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Laboratory of Nutrition, Faculty of Pharmacy, University Louis-Pasteur, Strasbourg, France
2 To whom correspondence should be addressed. E-mail: ingen{at}pharma.u-strasbg.fr.
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ABSTRACT |
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 TOP ABSTRACT LITERATURE CITED |
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KEY WORDS: • sulfur biology • S-containing compounds • food composition • sulfate balance • hyperhomocysteinemia • protein nutritional status • transthyretin
Starting from the pioneering studies performed many decades ago (1–3), our knowledge of N metabolism has gained continuing advances and remains a field of intense investigations. Large reviews have recently highlighted most known aspects of N turnover and amino acid (AA) metabolism (4–6), giving to the protein component of living organisms a sound theoretical basis and major clinical applications.
In contrast, S appears as the "forgotten" element. There exist several large volumes dedicated to all aspects of inorganic S chemistry (7,8), but the data available on the biological properties of the element are scanty. Despite the fact that S occupies the seventh rank among the most abundant elements of body tissues, not a single paragraph is devoted to S in health and disease by universally recognized position monographs. The last volume of Present Knowledge in Nutrition (9) completely ignores the importance of S in human nutrition, as do the last issues of the British Encyclopedia of Nutrition (10) and of the Apports Nutritionnels Conseillés pour la Population Française (11), which contains contributions from most French-speaking nutritionists. The same disinterest characterizes the renowned McCance and Widdowson's Composition of Food tables (12), whose successive editions overlook the S content of customary diets. The omission is all the more surprising because the quoted treatises develop large chapters assigned to most other elements, including those whose concentration in living beings is situated far below that of S. The reasons for such oversight probably lie in the unusual complexity of S derivatives in organic chemistry and, hence, the inaccurateness of true S requirements, endogenous distribution, and metabolic pathways.
The wind nowadays is blowing in the opposite direction. The renewal of interest in S appears as the result of improved analytic approaches and recent discoveries showing that S-containing components such as glutathione (GSH)3 and homocysteine (Hcy) are directly involved in immune and detoxifying processes (13) or in cardiovascular disorders (14) of significant public health importance. The fifth AAA Workshop offers a unique opportunity to throw further insights into the metabolism of both S and N and to reconcile domains that manifest close interactions from the earliest geological times of the planet.
N in the biosphere. N bears the atomic number 14 and constitutes 78% by volume of the earth's atmosphere. N has 8 valencies and oxidation states ranging from +5 (nitric acid, NO3H) to –3 (ammonia, NH4+). N is a rather inert gaseous element, poorly reactive and minimally soluble as NH4+ ion in ocean waters. Primitive marine algae are nevertheless able to utilize minor nitrogenous substrates for their growth. Large amounts of NO3H are produced by lightning and other electric discharges in the N atmosphere, leading to the formation of highly soluble nitrates (NO3Na, NO3K) dispersed in seawater or washed down onto earth by rain (15,16).
Atmospheric N may also be taken up by the nodules found on the roots of certain plants, belonging mainly to the legume family. This process of N fixation is performed by soil bacteria (Clostridium, Azotobacter, Actinomyces), which may be free-living or in symbiotic association with plants. Further converting processes occur at ambient temperature and usual atmospheric pressure, transforming the N accumulated by plant roots into NH4+. The reactions necessitate the bacterial activation of protein metalloenzymes (hydrogenases, nitrogenases) containing iron (Fe) and molybdenum as cofactors. NH4+ may then undergo nitrification processes producing nitrites and nitrates through the mediation of oxidizing microorganisms (16,17).
Atmospheric N is thus regarded as a primitive reservoir at the origin of NO3– and NH4+ salts in the biosphere. These primary inorganic substrates participate in the building of the nitrogenous compounds required for plant growth. NO3– and NH4+ precursor sources coexist within the N cycle, which is grounded on competitive relationships between nitrification and denitrification processes (16,18).
S in the biosphere. Sulfur is the second-row component in the group VIa of the periodic table of elements, situated just below O and bearing the atomic number 16. It has 8 valencies and oxidation states ranging from +6 (sulfate, SO42–) to –2 (sulfide, S2–). SO42– is a stable and inert oxyanion, whereas H2S constitutes the most reduced and reactive species. From the former to the latter, all intermediate states reveal increasing instability and reacting capacity. In the ocean waters, S mainly exists in the form of SO42– salts with a mean concentration reaching 0.0884%. Sulfates constitute 7.7% of the oceanic salts with extremely small variations in samples of different origin (7,8). On earth, S naturally occurs as a mixture of 4 isotopes, the most abundant being 32S (95.1%). S ranks number 15 among the elements of the earth's outer crust with a mean concentration of 0.048%. Elemental S is present on earth in its free state in the form of deposits of volcanic and sedimental origins. It reacts with almost all other elements, which explains the widespread distribution of S compounds in nature, mainly in the form of heavy metal sulfites (SO2–). N is one of the rare elements, together with iodine, gold, and noble gases, escaping this reactive property. This implies that the close companionship observed between S and N in living systems requires their prior transformation into bioavailable derivatives (8,19,20).
Various SO42– salts are found in the terrestrial environment, resulting from marine deposits or from the chemical conversion of sulfites and other S compounds into most oxidized oxyanions. These SO42– salts exhibit varying degrees of solubility and reactivity, being easily carried away by rain and ground water, and explaining extremely variable SO42– concentrations in springs, rivers, lakes, and wells. Table 1 reports the mean SO42– concentration measured in drinking waters usually consumed in several European countries. It is assumed that S has been crucially involved in the emergence of early life on earth and in the development of primitive respiratory exchanges under anaerobic circumstances. The supposed mechanism is grounded in the reduction of SO42– into H2S, a reducing process thought to be initiated by microorganisms and fungi, requiring considerable energy and the intervention of cytochromes and flavoproteins in support of electron exchanges (21,22). Because S is a homolog to O in the periodic table, it is assumed that bacteria living in anaerobiosis breathe S in the same sense that aerobic organisms are breathing O (7).
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The assimilation of N in plants starts with the uptake and conversion of NO3– to NO2– by root systems, a process highly regulated by a nitrate-reductase enzyme and followed by the transport of NO2– to the green leaves, where subsequent reductions to NH4+ occur (17). The predominant anabolic step takes place in the chloroplasts, where NH4+ is converted to glutamate through the mediation of 
-oxoglutarate and under the control of ATP-dependent activation of glutamate NH4-ligase (glutamate-synthase) (24). Via transaminating processes, glutamate becomes the principal provider of -amino N groups involved in the production of most other AAs, including that of the 8 indispensable AAs (IAAs) (25). In contrast to higher animals, plants possess the enzymatic equipment required for these syntheses. N assimilation appears as the direct consequence of photosynthetic reactions as ATP and reduced ferredoxin (S-rich protein comprising Fe-S clusters and displaying electron-carrier activity) are continuously generated in the chloroplastic cells (26). Figure 1 schematically outlines the main steps transforming elemental N and S of the biosphere into the basic organic molecules found in the plant organisms.
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Taken together, accumulation of N and S compounds in plant tissues follows comparable processes and displays close interrelationships. The cellular uptake of both NO3– and SO42– oxyanions exhibit multiphasic characteristics with Km and Vmax values modified following plant requirements. Tightly regulated mechanisms operate to prevent the excessive storage or shortage of N and S components to maintain narrowly fluctuating N:S molar ratios in plant tissues. In the case of SO42– overfeeding, the uptake and transport of the oxyanion is rapidly depressed (30) under the negative feedback control of Met and Cys concentrations (31). In the case of marginal SO42– deficiency, the activity of the rate-limiting ATP-sulfurylase is stimulated, driving a greater proportion of available SO42– into reducing processes (32) while inhibiting the activity of nitrate reductase and the rate of N sequestration (33). More marked SO42– deprivation leads to the slackening of plant growth, whereas total SO42– exhaustion ends at its complete standstill (34). Under optimal environmental and feeding conditions, the morphological growth of plants is an uninterrupted, seasonally graded process, as they continue to take up N and CO2 from the atmosphere and NO3–/SO42– from the soil throughout a large part of their life span. Despite an almost 3000-fold variation in the medium SO42–, there exists a striking constancy in the rate of Met and Cys production (35). These 2 SAAs represent the bulk (>90%) of all plant protein-bound S molecules (19), whereas the proportion of free Met and free Cys is normally low and relatively constant (35). It is of interest to note that a minor fraction of plant Cys participates in the synthesis of the GSH tripeptide, the most important non-protein-bound thiol, which is mainly located in the chloroplastic cells (19). In contrast with both SAAs, the GSH pool found in plants is highly sensitive to the provision of precursor substrates, increasing significantly on high NH4+ and Cys supply but becoming depleted after their withdrawal from the milieu (19). The plant GSH fulfills many roles in plant physiology, serving as a storage form of Cys, scavenging free radicals, and maintaining the redox state of cells (19,20). Table 2 collects the main N and S constituents of some currently consumed plant foodstuffs characterized by molar S:N ratios usually ranging from 1:20 to 1:35. As a corollary, most plants used for human consumption reveal limiting SAA content in relation to human requirements, which explains why plant-eating populations incur the risk of permanent N and S deficiencies. Soy products used as staple food by many vegans constitute a notable exception. Because of its unusual N and SAA richness, soy adequately fulfills human tissue requirements and does not necessitate supplemental Met to reach the biological value of animal foodstuffs (38).
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N is, after H, O, and C, the fourth most abundant element in mammalian tissues, totaling 64 mol (1800 g) in a reference man weighing 70 kg (39). N is mainly sequestered within the lean body mass (LBM), which accounts for most of the TBN of the body, assuming a mean 6.25 ratio of protein to N. The total protein mass of a normal individual represents about 15% of his body mass, thus corresponding to 10.5 kg of the body weight. TBN fluctuates within narrow limits (r = 0.96) with total body K (TBK), the main intracellular cation (40,41). Several studies, using metabolic and analytic approaches including isotope dilution and in vivo prompt 
neutron capture (42,43), have concluded that TBN is sequestered within 3 distinct compartments, the first being a very low-energy-metabolizing and poorly exchangeable pool (about 30% of TBN) (42,44). The bulk of TBN partitions into 2 exchangeable pools, namely the metabolic pool made up of tissues characterized by rapidly turning over proteins (liver, gut mucosa, kidneys, spleen, pancreas, and thymoleukocytic cells) and the structural pool comprising organs with slower turnover rates (muscle mass and skin) (45). Under steady-state conditions, the overall TBN turnover approximates 3% per day or about 300 g proteins degraded and regenerated every day (45). The fractional synthesis or renewal rates of the liver and of the gut mucosa protein pools are 10 to 20 times more rapid than that of the skeletal muscle mass. However, because of the size of the musculature, which corresponds to 37% of the body mass (46), it makes at least an equal contribution in absolute terms to the daily turnover of proteins in the body, as compared with the liver and intestine (45). The daily maintenance of TBN is achieved through coordinated changes in the rates of whole-body protein turnover as maturation of the tissues, interorgan AA exchanges, oxidation, and regeneration are tightly regulated processes throughout the entire life span, offering sex- and age-related specificities from birth to death (47–49). Taken as a whole, TBN is minimal in neonates and increases linearly along superimposed straight lines in preadolescent children of both sexes (50,51). A sexual difference occurs at the onset of puberty, with higher TBN values recorded in male young adults compared with their female counterparts, as a result of a deeper androgenic hormonal status correlated with a larger proportion of muscle tissue (45,48). This sex-related difference is maintained in the form of plateau levels during full sexual maturity (45,48). Starting in the 60s, TBN progressively declines over time, revealing a steeper slope in elderly men that reflects a relatively more rapid deterioration of their muscle mass (45,48). As a result, the earlier TBN sexual difference disappears by about the age of 70 y (45). The sex- and age-related peculiarities described for TBN are closely paralleled by the time course recorded for TBK and creatininuria (52), the latter being a marker of the fractional catabolic rate of skeletal muscle, confirming that these biological parameters faithfully follow the shape disclosed by total LBM and more precisely by the muscle mass. The serial measurement of plasma TTR is a sensitive and useful marker of whole body protein nutritional status (53). The time course of TTR from birth to death is indeed closely superimposable on TBN in subjects of both sexes (45,54), conferring on this plasma marker the unique property of reflecting the actual level of protein reserves remaining bioavailable in both exchangeable N pools of the body.
S in the animal kingdom. S is the seventh most abundant element in the tissues of higher vertebrates. The body of the reference man weighing 70 kg contains 140 g S (4400 mmol), the same order of magnitude as that of TBK (39). Both S and K elements thus represent 0.2% of body weight under steady-state conditions. The mean molar S:N ratio measured in mammalian (rat, dog, cattle, man) tissues is 1:14.5 (39,55–57).
There exist 2 principal dietary sources supplying S-containing compounds in human nutrition, namely, liquids and solid food products. The former vehicle is by far the main SO42– dietary variable. Table 1 shows that the concentration of SO42– in samples of natural drinking water may vary considerably from <2 mg to >1 g/L, meaning a ratio exceeding 1:500 in the most extreme situations. Food provides the body with organic S, mainly in the form of Met and Cys. Solid food items naturally possess little other S-containing ingredients with the exception of some plants belonging to the Brassicae class of vegetables (cabbage, cauliflower, turnip), which are a source of S-glucosinates (58). Some commercial products (bread, sausages) may be enriched during processing with sulfite or sulfate salts (58) used as preserving or flavoring agents. Table 3 reports the water content, energy density, Met, Cys, N, and S concentrations and S:N ratios of some currently consumed animal foodstuffs. The data indicate that these foods exhibit relatively stable S:N ratios that are close to those defining the chemical composition of mammalian tissues. By comparison with Table 2, it is apparent that plant products, taken together, are characterized by more heterogeneous energy density and contain lower concentrations of N and S. It must also be noted that Met may constitute, together with lysine, 1 of the 2 limiting IAAs harming the biological value of plant products (59) whose nutritional quality may be further reduced by lower intestinal digestibility and by food processing (60).
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True human N and Met recommended dietary allowances (RDAs) are defined as follows: the reference adult man weighing 70 kg requires the minimal safe protein level of 0.75 g/kg or 52 g on a daily basis (6,68). The recommended level of safe Met intake ranges from 13 mg/kg (6,68) to 16 mg/kg (69), meaning 910 to 1120 mg/d. In Western societies, the average intake of SAAs by the reference man amounts to 3.6 g/d (two-thirds as Met and one-third as Cys) or 26 mmol/day (70), which corresponds to 1100 mg SO42– equivalents. Met has a rapid and efficient intestinal absorption rate (90 to 100% of intake) (71). SAAs entering extracellular water space (ECW) from dietary sources rejoin the flux of 70 mmol SAAs turning over every day (body proteins release about 32 and 38 mmol/day of Met and Cys in free form, respectively) (72), forming together an endogenous pool of about 100 mmol SAAs. Under steady-state conditions, >90% of these are recycled into neosynthetic processes to replace proteins having been broken down. A minor fraction estimated at 7 or 8% of the turning-over pool undergoes oxidative degradation along the transsulfuration pathway before leaking SO42– in its distribution space. The minimal obligatory losses (see below) measured in the urinary output indicate that Western standard dietary practices ensure a large margin of safety in terms of SAA coverage. Under conditions of dietary Met loading or shortage, the respective proportion of SAAs undergoing neosynthetic or degradative pathways may be substantially modified.
Dietary SO42– is completely absorbed by the intestinal tract whithin <2 h (73), as fecal losses are negligible in normal subjects at all doses (74). Animal experiments have demonstrated that crossing the intestinal barrier requires an active and Na+-dependent process (75). Plasma inorganic SO42– manifests a circadian rhythm with values reaching nadir levels in the morning and maximal levels in the early evening (76). The oxyanion does not bind to plasma protein and diffuses readily in its endogenous distribution volume, which coincides with the ECW (77). Radiolabeled 35SO42– seems therefore the most reliable method for assessing the ECW volume that has been estimated at 23% of body weight (about 16 L in our reference man), provided that zero-time extrapolation correction for renal excretion is made (78). Because the mean SO42– serum concentration (360 µmol/L or 12 mg/L) (73) equilibrates rapidly with extracellular fluids, the SO42– pool confined within the ECW volume amounts to about 200 mg. The SO42– sequestered in the ECW compartment has a relatively short T
ranging from 4 to 9 h in healthy individuals (79), explaining why at least 80% of the oral dosage is recovered in 24-h urine (73). Intravenously infused SO42– salts are nearly completely excreted in the urinary output over 24 h (80,81). This efficient renal clearance is consistent with free glomerular filtration and low tubular resorption capacity (80), preventing the excessive accumulation of SO42– in body fluids. Under normal conditions, the concentration of SO42– oxyanions excreted in free form constitutes about 90% of total sulfaturia (74). A minor part consists of SO42– bound derivatives, mainly esters of phenol, indole, and p-cresol, that are generated by the bacterial degradation of aromatic AAs in the intestinal tract (82,83). Figure 2 tentatively quantifies the distribution and balance of SO42– resulting from dietary intake and urinary output.
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Although the tissue distributions of S and N compounds are not strictly aligned, and minor fractions of TBS participate in the structure of lipid and carbohydrate molecules, a growing body of data suggest that TBS, as well as TBN, partitions into 3 distinct body compartments. The first is a nonmetabolic, poorly exchangeable pool characterized by an unusual richness in S-S covalent bridges between adjacent polypeptide chains, confering hardness and rigidity to the protein structure. A good example is given by keratin molecules found in hair, nails, and horny integuments. Collagen, connective tissue, cartilage, tendons, and fascia also belong to this compartment, which seems poorly influenced by dietary and metabolic alterations. It is likely that this compartment corresponds to about one-third of TBS with sex- and age-related peculiarities and significant interspecies differences. Using 35SO42– as a tracer of tissue distribution, Australian researchers were able to demonstrate that the relatively higher S requirements of sheep over cattle were caused by their increased demand in wool production (90). The last 2 pools are exchangeable, offering many similarities with both the above-described metabolic and structural pools for TBN. The long-lasting labeling of rat tissues with 35S-Met has allowed identification of a rapidly turning over pool (7% per day with T of 10 d) and a slowly moving pool (0.4% per day with T of 175 d) (91). The former consists in labile compounds and non-protein-bound thiols, mainly GSH, present at high levels in all mammalian cells with maximal concentration measured in the liver (92,93). The latter notably comprises the muscle mass, which possesses lower GSH reserves (93) somewhat compensated for by high amounts of Tau, the most abundant intracellular free AA in mammalian tissues (94,95).
N and S in protein-depleted states.
Under steady-state conditions, protein synthesis and cell growth rates are maintained at levels consistent with nutrient availability. AA supply results in phosphorylation of the ribosome-associated S6 kinase, whose activation favors the translation of mRNA species that encode ribosomal proteins (96). A salient feature of adaptive changes developed under conditions of dietary protein restriction or excessive protein losses is the down-regulation of most N-dependent metabolic reactions. Protein synthesis falls, and protein breakdown is altered in most organs belonging to both metabolic and structural N pools (45). The complex and coordinated salvage mechanisms implicate direct modulation of protein synthesis at the transcriptional level by reduced AA bioavailability (97), changes in enzyme concentrations or activities in response to protein shortage (98), regulation of the ubiquitin-proteasome pathway by dietary manipulation (99), and stimulation of lysosomal proteolysis (100). The net result of these adaptive changes leads to the linear decline in the urinary excretion of endproducts of N catabolism over widely varying protein intakes (101), and the nutritional importance of some non-IAAs such as glutamate may be upgraded to become a key supplier of -amino N groups (102).
In overt cases of malnutrition, the depletion of N from body reserves has been initially documented after the chemical analysis of the carcass of famine victims (103). In acute starvation experiments, the largest loss of protein per unit weight originates from the liver metabolic pool, with 40% of its protein content being degraded during a 7-d fast (104). Under more chronic circumstances, organs and tissues are affected to different extents depending on the duration of protein shortage (105). The shrinking of exchangeable N pools is documented in most conditions of dietary deprivation or intestinal malabsorption, such as in weight-losing regimens (106), anorexia nervosa (107), and cystic fibrosis (108). In most starvation experiments, the urinary excretion of N and S catabolites remain highly correlated (88,109,110), pointing to concomitant tissue depletion rates of N and S stores. Animal (56,64) and human (110) studies nevertheless indicate that a relatively greater proportion of S over N is leaked, suggesting that the S exchangeable pools might be more responsive to dietary protein restriction. This early mobilization of S compounds could be related to the presence of labile non-protein-bound thiols in most mammalian tissues, with large interspecies analogy (93). The liver is by far the organ containing the largest concentrations of GSH in free form (about 50% of total SH groups), whereas the free GSH content in the muscle mass usually amounts to 3–4% of SH values (93). Most other organs occupy intermediary positions between liver and musculature in terms of free GSH richness (93). The high reactivity and rapid depletion of GSH stores measured in plasma (111,112), leukocytes (112), intestine (113,114), and liver (92,114) are fully documented. Some studies speculate that at least 2 different GSH pools might coexist in the liver tissue, displaying distinct turnover rates in response to starvation–refeeding experiments (92). The biological half-life of hepatic GSH is estimated at about 4 h (115). In animal species, as in the plant kingdom, GSH tripeptide is regarded as a readily available reservoir of Cys (92,114), fulfilling a number of immune and detoxifying properties. In contrast, erythrocytes and muscle GSH reserves are poorly responsive to dietary manipulation (114). GSH in red blood cells manifests a slow T of about 4 d (116).
Whatever the alterations of dietary compounds, most animal and clinical studies converge on the conclusion that both methioninemia and sulfatemia maintain remarkably stable plasma concentrations. In the case of persisting deviation from recommended dietary patterns, the ultimate choice made by the body is to favor Met stability at the expense of SO42– values. The preservation of Met homeostasis may be partly ascribed to the combined effects of high intestinal and renal recovery rates, minimizing fecal and urinary losses (61). Moreover, subtle competitive interactions between the transsulfuration and remethylation pathways of Hcy contribute to the adaptation of the tissue requirements for Met in response to a broad spectrum of protein intakes. The relative importance of both metabolic pathways is regulated by S-adenosylmethionine (SAM), a dephosphorylated molecule resulting from the condensation of Met with ATP. SAM works as an allosteric inhibitor of methylenetetrahydrofolate reductase (117) and as an allosteric activator of cystathionine-ß-synthase (CßS) (118). Human CßS is a 63-kDa enzyme composed of an N-terminal protoporphyrin-containing domain, a catalytic site that binds pyridoxal-5'-phosphate (PLP), and a C-terminal sequence standing under SAM control (119). To date, it has been established that folate (120) and cobalamin (121) shortage are both capable of depressing the activity of Met synthase, causing the downstream accumulation of Hcy in extracellular fluids. Moreover, dietary PLP deprivation may impair the activity of CßS, the rate-limiting enzyme initiating the transsulfuration cascade (122), thereby promoting the upstream sequestration of Hcy. Stepwise multiple regression analysis has concluded that deficiencies of the 3 water-soluble vitamins, working together, do not account for >40% of the variance in total Hcy blood concentrations (123,124). In an attempt to fill the gap between the already recognized causal factors of undesirable hyperhomocysteinemia and the observed epidemiologic findings, we postulate that the fluctuations of TBN might operate as a major determinant of Met and Hcy homeostasis (61). This hypothesis implies that the acquired blockade of CßS is an N-sensitive step allowing the conservation of Met homeostasis under conditions of nutritional or inflammatory disorders. Met indeed participates in the synthesis and structure of many S-containing compounds and contributes to a myriad of metabolic and catalytic reactions of vital importance that are briefly summarized in Table 4. The crucial nutritional and metabolic roles played by Met in mammalian physiopathology are highlighted by N balance studies performed on at least 3 different animal species (rat, pig, cattle). Met is recognized as the most limiting of all IAAs because its complete withdrawal from otherwise normal diets causes the greatest rate of body N loss, nearly equal to that generated by a protein-free regimen (64,125,126).
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The downsizing of TBN stores resulting from conditions of protein restriction is well identified by the measurement of TTR (45). It is indeed shown that the liver of rats submitted to protein deprivation manifests a specific decrease in the abundance of TTR nuclear transcripts that is correlated with a decrease in TTR mRNA (134). The data point to a depressed transcriptional regulation of TTR synthesis directly mediated by the dietary shortage of some IAAs (134). TTR is a tetrameric molecule unusually rich in tryptophan (135) that constitutes the smallest of all IAA pools in the tissues of higher vertebrates, likely operating as an early limiting factor for the synthesis of compounds in great demand for this essential AA (53).
N and S in stressful disorders.
A stressful condition of any cause is typically characterized by a cascade of cytokine-induced metabolic reactions aimed at setting up immune and repair responses. Proinflammatory cytokines, mainly interleukin-1, interleukin-6, and tumor-necrosis factor (TNF), and counterregulatory hormones (cortisol, glucagon, catecholamines, somatotrophin) create a stage combining insulin refractoriness and euthyroid sick syndrome to generate overall down-regulation of metabolic processes based on the selective mobilization of lipid reserves from healthy tissues (136). By contrast, damaged tissues are up-regulated and switched toward the preferential utilization of available AAs for the production of acute-phase proteins and wound healing with anaerobic glycolysis as the prevailing energy source (136). Taken together, cytokine and hormonal effector molecules thus operate in concert or antagonistically to drive the body economy toward a dichotomous partitioning whose metabolic responses may be impaired in patients affected by poor protein nutritional status. These events lend support for the nutritionally dependent adaptive dichotomy (NDAD) concept (137).
The course of stress is associated with obligatory urinary losses of N substrates generated by an increase in protein catabolism overpowering an increase in protein synthesis (138). The leakage of urea largely predominates, but the presence in the urine of creatinine and 3-methylhistidine, contributing to a net body negative N balance, indicates that muscle tissue breakdown participates in the stress reaction (139). The data are well documented in a number of acute and chronic stressful conditions such as trauma (140), surgery (141), sepsis (142), AIDS (143), and cancer (42). N hypercatabolic losses are accompanied by concomitant S urinary losses, both culminating within 3 to 6 d from the time of injury (140). Direct measurement of N and S excreted by adult patients suffering from bone fractures results in a calculated S:N score of 1:17.7 (140), again very close to the mean chemical composition of human tissues (39) and pointing to parallel tissue depletion rates. Not surprisingly, CßS, which occupies a central branch point for Hcy remethylation and transsulfuration, is highly responsive to a large set of nutritional, hormonal, and inflammatory influences (144). Its N-terminal heme-binding domain may work as a PLP-dependent redox sensor (119), and its C-terminal sequence may be subjected to TNF proteolytic cleavage, releasing a truncated segment and becoming less sensitive to SAM stimuli (145). Divergent consequences follow from these molecular investigations; the redox regulation of CßS activity could help stimulate the overproduction of Cys and of antioxidative molecules, mainly GSH (146,147). Other studies, however, defend the opposite opinion of depressed transsulfuration rates in some diseased states such as cirrhosis (71) or burn injury (148). In overt cases of protein malnutrition with documented liver dysfunction, it has been suggested that the impairment of CßS activity could be directly correlated with the level of residual TBN stores, irrespective of PLP, SAM, and redox status (61). The concept is in good agreement with a general rule in protein metabolism, showing that a diet lacking 1 IAA results in the inhibition of its catabolizing enzymes to prolong its conservation within endogenous pools (149). Such an enzyme-induced sparing effect is substantiated for the 7 other IAAs whose disposal fails to meet optimal tissue requirements (61).
Healthy individuals subjected to stressful conditions usually exhibit cytokine-induced suppression of TTR synthesis (150). Consequently, plasma concentrations fall until day 3 to 6, a nadir paralleling that of TBN and coinciding with the peak of nitrogenous compounds excreted in the urine (45). The amplitude of these reciprocal changes is proportionate to the severity of injury. Inadequate nutritional management, multiple injuries, severe sepsis, or metabolic disturbances result in persistent N losses and subnormal TBN and TTR values. Attenuation of stress and/or nutritional rehabilitation allows restoration of both TBN and TTR values following parallel slopes (45).
Clinical implications. The above-reported data have documented that both TBN and TBS constitute closely interwoven pools subdivided into 1 poorly exchangeable compartment and 2 compartments slowly and rapidly reacting to alterations of protein nutritional status. In all conditions characterized by a negative body N balance, such as in dietary restriction or cytokine-induced hypercatabolic losses, N and S endogenous pools manifest parallel tissue depletion rates. From physiopathogenic standpoints, both adaptive responses are clearly distinct in nature but nevertheless converge toward a common impoverishment of N reserves affecting the metabolically responding organs. Both metabolic and structural N pools become depleted in varying proportions depending on the nature, intensity, and duration of the causal factor. Conversely, restoration to normal of protein malnutrition or inflammatory burden results in parallel N and S accretion rates.
The magnitude of TBN depletion determines the clinical course and outcome of both malnourished and injured patients. Several biomarkers are recommended as working tools allowing the detection and follow-up of diseased individuals. Direct analysis of TBN (108) or TBK (151), accurate indicators of protein status, are not applicable on a routine, clinical care basis. A close relationship has been established between body composition and muscle function (107), hence the proposal to assess protein status using anthropometric measures (152,153). These approaches appear useful in chronic circumstances but are poorly informative in short-term alterations. Finally, the serial measurement of TTR appears to be a simple, robust, and dynamic indicator of TBN fluctuations in both protein-depleted and stressed patients (45). TTR fulfills a unique position in assessing the actual protein nutritional status, in monitoring the efficacy of nutritional rehabilitation, and in predicting the patient's outcome (45). TTR could also serve as a useful index to identify the N component responsible for the maintenance of Met homeostasis, especially when the vitamin supply seems adequate or is only marginally deficient. There exists an abundant literature describing the high prevalence of hyperhomocysteinemia in plant-eating individuals (154,155). Most of these studies incriminate vitamin B12 deficiency as the causal factor but fail to utilize appropriate markers for detecting the potentially responsible protein component, casting some doubt about the validity of their interpretation. Our view is that hyperhomocysteinemia principally results from a state of chronic N and Met deprivation undermining the TBN reserves of exposed populations and whose magnitude is best appraised by the inverse correlation linking declining TTR values to rising Hcy concentrations (61,129; Fig. 3). This harmful situation may become even worse with the occurrence of any inflammatory burden further deteriorating the negative N balance (142) or by its coexistence with a cobalamin –deficiency as frequently encountered in low-income countries (156).
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FOOTNOTES |
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3 Abbreviations used: IAA, indispensable amino acid; GSH, glutathione; LBM, lean body mass; NDAD, nutritionally dependent adaptive dichotomy; TBK, total body potassium; TBN, total body nitrogen; TBS, total body sulfur; TTR, transthyretin.
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LITERATURE CITED |
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