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* Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, AB, Canada, T6G 2P5;
The Department of Biochemistry, Memorial University of Newfoundland, St. John’s, NL, Canada, A1B 3X9;
** Department of Animal Science, University of Manitoba, Winnipeg, MB, Canada, R3T 2N2;
The Research Institute, The Hospital for Sick Children, Toronto, Canada;
Department of Paediatrics and
Department of Nutritional Sciences, University of Toronto, Toronto, ON, Canada, M5G 1X8
3 To whom correspondence should be addressed. E-mail: ron.ball{at}ualberta.ca.
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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70% of the enteral requirement when measured in the presence of excess dietary cysteine (P < 0.05). A comparison with our previous studies in which methionine was the only source of sulfur amino acids shows that the addition of dietary cysteine reduces the methionine requirement by 40% in both enterally and parenterally fed neonatal piglets. Therefore, dietary cysteine is equally effective in sparing dietary methionine whether fed enterally or parenterally.
KEY WORDS: • cysteine • cysteine sparing • indicator amino acid oxidation • methionine • total parenteral nutrition
Whether cysteine is a conditionally indispensable amino acid in either neonates or those administered total parenteral nutrition (TPN)3 remains controversial. In adults, adequate cysteine is synthesized from methionine via the transsulfuration pathway (1). Cysteine is therefore considered to be dispensable in adults. However, de novo cysteine synthesis may be limited in neonatal animals due to the slow maturation of the enzyme cystathionase (EC 4.4.1.1) (2–4). Moreover, cysteine synthesis may be further limited in parenterally fed neonates (5) and adults (6); however, the mechanism responsible for this is unknown. Generally, cysteine is thought to be semiessential during these periods because of the low plasma cysteine that is associated with TPN feeding, but few studies have evaluated enzyme activity or noted difficulties in maintaining growth. We previously reported the requirement for methionine, in the absence of dietary cysteine, to be 30% higher in enterally compared with parenterally fed neonatal piglets (7). This finding suggests that the small intestine utilizes approximately one third of the dietary methionine. In that study, total sulfur amino acids (TSAA) were supplied by methionine alone. The extent to which dietary cysteine can spare the requirement for methionine in neonates must be defined to help determine the optimal concentration of cysteine in diets for young pigs and in pediatric parenteral solutions. In addition, the use of cysteine to meet part of the SAA requirement during TPN would avoid the toxic effects of excess dietary methionine (8). Information obtained from neonatal piglets will provide the groundwork for future clinical metabolic studies investigating the optimum intake and ratio of methionine to cysteine in human neonates.
Numerous studies in orally fed animals showed that dietary cysteine can replace part of the methionine requirement. Estimates of cysteine sparing range from 40 to 70% in growing pigs (9–15), 50–55% in chicks (16–19), 17–90% in humans (20–24) and 50–65% in rats (25–28). Other data demonstrated that increases in dietary cysteine downregulate or inhibit cystathionine ß synthase (CBS; EC 4.2.1.22), thereby diminishing transsulfuration and increasing methionine retention (28–31). A set of studies (32–36) used methionine and cysteine tracers to investigate SAA metabolism in humans, and all determined that cysteine had no sparing effect on the requirement for methionine. In each of these human studies, methionine and cysteine were provided at the "requirement level" as recommended by the FAO/WHO/UNO (37). However, recent studies from our group showed this level of SAA intake to be deficient for humans (24,38). The lack of cysteine sparing observed in the earlier human studies may have been due to an inadequate supply of dietary methionine because cysteine can spare methionine only after the obligatory methionine requirement is met. Using double-labeled methionine, DiBuono et al. (39) showed that when a higher methionine intake was provided, dietary cysteine reduced conversion of methionine to cysteine, thereby sparing the dietary requirement for methionine. This research (24,38,39), demonstrating the methionine sparing capacity of cysteine during oral feeding, now brings the human research into agreement with all of the animal research. However, to the authors’ knowledge, cysteine sparing has not been estimated previously during parenteral feeding.
We successfully applied the IAAO technique to measure the amino acid requirements in piglets fed enterally and parenterally (40–42). Subsequent studies in human neonates confirmed that findings in the piglet model are valid in parenterally fed human infants (43). In the present study, the objective was to determine the mean methionine requirement when excess dietary cysteine was provided, in neonatal piglets administered TPN or enteral nutrition by employing the IAAO technique. Subsequent comparison of these requirements with our previous data (7) in which no dietary cysteine was provided would enable calculation of the cysteine sparing capacity. We hypothesized that the capacity of cysteine to spare the methionine requirement would be 50% in both routes of feeding.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The Faculty of Agriculture, Forestry and Home Economics Animal Policy and Welfare Committee at the University of Alberta approved all procedures in this study. Male Landrace/Large White piglets (n = 28; Genex Swine Group) were obtained from the University of Alberta, Swine Unit (Edmonton, Canada). Piglets were transported to the Metabolic Unit at the University of Alberta. The piglets were weighed and then anesthetized with acepromazine (0.5 mg/kg; Atravet; Ayerst Laboratories, Montreal, Canada) and ketamine hydrochloride (22 mg/kg; Rogarsetic; Rogar STB, Montreal, Canada) and maintained during surgery with 1% halothane. All piglets were then fitted with two venous catheters (Ed-Art, Don Mills, Canada) using the modified methods of Wykes et al. (44); in enterally fed piglets (n = 14), gastric catheters were inserted using the method of Rombeau et al. (45). In all pigs, an infusion catheter was inserted into the left jugular vein and advanced to the superior vena cava just cranial to the heart, and a sampling catheter was inserted into the left femoral vein and advanced to the inferior vena cava just caudal to the heart. After surgery, incision sites were treated with a topical antibiotic (Hibitane Veterinary Ointment: Ayerst Laboratories) and an analgesic (0.1 mg/kg Buprenex, Buprenorphrine HCl, Reckitt and Colman Pharmaceutical, Richmond, VA) was given i.m. immediately and again 8 h postsurgery. Piglets were then put into cotton jackets, which secured the tether to the piglets. The tether is part of a swivel-tether system (Alice King Chatham Medical Arts, Los Angeles, CA) that enables pigs to move freely while being administered a continuous dietary infusion, ensuring that the catheters to do not become tangled or occluded.
Animal housing.
Piglets were housed in individual circular cages, 75 cm in diameter, and toys were added to enhance their environment. The animal rooms were maintained at an ambient temperature of 21–27°C, with supplemental heat supplied by heat lamps. Lighting was on a 12-h light:dark cycle.
Diet regimen.
Elemental diets were provided as continuous infusions by pressure-sensitive infusion pumps. Piglets were fed 15 g amino acids/(kg · d) and 1.1 MJ metabolizable energy/(kg · d) with glucose and lipid (Intralipid 20%, Fresenius-Kabi, Stockholm, Sweden) each supplying 50% of nonprotein energy intake. The base amino acid profile of the complete elemental diet fed during adaptation (d 0–5) was described previously (7). The amino acid profile was based on human milk protein (Vaminolact: Fresenius-Kabi, Stockholm, Sweden) except for phenylalanine and tyrosine, which were provided at their estimated safe levels of intake (46,47) and arginine, which was provided at 1.2 g/(kg · d) (48). The majority (73%) of tyrosine was provided as the dipeptide glycyl-tyrosine (44). Diet infusion rates were adjusted daily after the piglets were weighed. Vitamins were supplied in a commercial solution, MVI Pediatric (Rhone-Poulenc Rorer Canada, Montreal, Canada), which was added to the diet immediately before feeding. The cofactors involved in the transsulfuration pathway, vitamin B-12, choline, B-6 and folate, were in the MVI solution at 115% of requirement (49). Piglets also were given a mineral solution including zinc, copper, manganese, chromium, selenium and iodide at 200% of the NRC (49) recommendation for piglets.
TPN was initiated immediately after surgery and increased to full infusion rates [13.5 mL/(kg · h)] by the end of d 1 (7). Complete TPN was continued until 1800 h on d 5. Piglets were then randomly allocated to one of the test levels of methionine [parenterally fed: 0.025, 0.05, 0.1, 0.15, 0.2, 0.3, 0.5 or 0.8 g·/(kg · d); enterally fed: 0.025, 0.05, 0.15, 0.25, 0.35, 0.45 or 0.6 g/(kg · d)]. These dietary methionine concentrations represent a range of 1.65–55 and 1.65–40 mg/g of total amino acids for parenterally and enterally fed piglets, respectively. Cysteine was provided as L-cysteine (99% by analysis) in all test diets, providing 0.55 g/(kg · d). All test diet solutions were made isonitrogenous by dissolving the appropriate quantity of L-methionine (at test level) and L-alanine (to maintain isonitrogenous solutions) in 75 mL of sterile water; these solutions were then transferred into infusion bags containing 675 mL of concentrated TPN solution without any L-methionine or L-alanine. The solutions were filtered with a 0.22-µm filter (Millipore, Milford, MA). Due to the highly unstable nature of L-cysteine in aqueous solutions, test diets were made immediately before infusion of the diet. Piglets were fed the test diet from 1800 h on d 5 until the completion of the oxidation study on d 6. Subsequently, piglets were returned to the complete diet for 24 h. At 1800 h on d 7, piglets were randomly assigned to a second test diet and a second oxidation study was performed on d 8. Previously, we determined that a second IAAO study 48 h after the first study produces data not different from the first study; therefore, two studies can be conducted to reduce the number of animals required for an entire IAAO requirement study (Brunton and Ball, unpublished data).
Tracer infusion, 14CO2 collection and analytical procedures.
Phenylalanine oxidation and flux were determined by a primed [186 kBq (5 µCi/kg)], constant intravenous infusion {130 kBq [3.5 µCi/(kg · h)]} of a tracer solution containing 92.8 MBq (2.5 mCi)/L of L-[1-14C]phenylalanine [200 MBq (54 mCi/mmol; American Radiolabeled Chemicals, St. Louis, MO)]. The constant infusion was 4 h to reach a plateau in both blood and breath labeling. Details of the infusion protocol, 14CO2 collection and blood collection procedures were described previously (46). After the infusion on d 8, piglets were killed by injection of 1000 mg of sodium pentobarbital into a venous catheter.
Reversed-phase HPLC with the use of phenylisothiocyanate derivatives was used to measure plasma amino acids. Collection and liquid scintillation counting of radioactive fractions to determine the specific radioactivity (SRA) of plasma phenylalanine and tyrosine were completed as previously described (46). The calculations for intake, oxidation, flux, nonoxidative disposal, release from protein breakdown, balance and percentage of dose oxidized were as reported previously (46). SRA for both plasma phenylalanine and tyrosine during the IAAO study were plotted and plateau values were calculated as the mean SRA at plateau. Plasma concentrations are represented as the mean concentration at each test methionine level.
Determination of plasma cysteine.
Plasma cysteine was analyzed according to the reversed-phase HPLC method of Araki and Sako (50) with modifications as suggested by Gilfix et al. (51). The inter- and intra-assay CV were <2%.
Statistical analyses.
A completely randomized design, with methionine intake serving as the main treatment effect, was used in this experiment. The effects of day of IAAO study (d 6 or 8) and weight at study were not significant (P > 0.10) using an ANOVA (SAS/STAT, version 8.1, SAS Institute, Cary, NC). If P-values were <0.05 for the F-value of the ANOVA model, significant differences between treatments were determined using the Student Newman Keuls multiple comparison procedure. Determination of the methionine requirement was performed using a two-way linear crossover model, as described previously (52,53). Regression analysis variables were dietary concentration of methionine as the independent variable and percentage of dose oxidized and phenylalanine oxidation as the dependent variables. The 95% CI for the estimation of a safe level of intake was also determined.
To compare the breakpoints, or requirement estimates, between parenterally and enterally fed piglets, we treated the breakpoint as a sample mean and used the pooled two-sample t procedure (54). Subsequently, we compared the present results with those from a previous study (7) to calculate the sparing capacity of cysteine within route of feeding (Table 1). On the basis of the assumption that the subjects were derived from the same population and identical procedures were used, the true variance was assumed to be the same. Calculation of pooled variance was used to average each sample variance with weights equal to its df. Therefore, pooled variance for the two groups was determined and used to test whether the two breakpoints were different using a pooled two-sample t procedure.
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Parenteral and enteral methionine requirement.
Breath 14CO2, plasma phenylalanine SRA and plasma tyrosine SRA reached a plateau 2 h after the initiation of the primed, constant infusion in all pigs. The ratio of plasma tyrosine SRA/ plasma phenylalanine SRA did not differ among diet treatments; thus, the rate of conversion of phenylalanine to tyrosine did not affect the breakpoint estimate.
Parenteral methionine requirement.
Phenylalanine flux, intake, nonoxidative disposal and release from protein did not differ (P > 0.05) among dietary treatments (Table 2). The similarity in flux among dietary treatments indicates that the differences observed in phenylalanine oxidation reflect a shift in partitioning between oxidation and protein synthesis. Furthermore, the lack of difference in Tyr:Phe SRA indicates that when dietary tyrosine was in excess, phenylalanine was partitioned between nonoxidative disposal and oxidation in proportion to the changes in protein synthesis (46,55). Phenylalanine oxidation, expressed as the percentage of the dose oxidized was affected by methionine intake (P = 0.03, Table 3) and tended to differ (P = 0.06) when expressed as absolute oxidation (Table 2). As methionine intake increased from 0.05 to 0.18 g/(kg · d), phenylalanine oxidation (% of dose) decreased (P < 0.05). Further increases in methionine intake did not affect phenylalanine oxidation (P > 0.05, slope was not different from zero, Fig. 1).
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Methionine intake did not affect the plasma free amino acid concentrations of methionine and cysteine (P > 0.05, Table 1 and Fig. 2), similar to a previous study in which we altered dietary methionine (7). Alanine was used to make the diets isonitrogenous; therefore, the intake of alanine and plasma alanine concentrations decreased as dietary methionine increased (Table 4). The plasma concentrations of tyrosine, taurine, serine, leucine, isoleucine, valine, histidine, arginine, proline, citrulline, glutamate, glutamine and asparagine decreased significantly as dietary concentrations of methionine increased (Table 4). Plasma concentrations of phenylalanine decreased with diets containing 0.025–0.3 g methionine/(kg · d) and then did not change with further increases in methionine intake.
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Phenylalanine flux, intake, nonoxidative disposal and release from protein did not differ (P > 0.05) among dietary treatments (Table 5). Phenylalanine oxidation, expressed as the percentage of the dose oxidized was affected by methionine intake (P < 0.005, Table 6). As methionine intake increased from 0.1 to 0.36 g/(kg · d), phenylalanine oxidation (% of dose) decreased (P < 0.01, Fig. 3). Further increases in methionine intake did not affect phenylalanine oxidation (P > 0.05, slope was not different from zero; Fig. 3). The breakpoint estimate for phenylalanine oxidation rate [requirement = 0.21 g/(kg · d), 95% CI: 0.02–0.37 g/(kg · d)] was similar to that for phenylalanine oxidation rate as a percentage of dose [Fig. 3; requirement 0.25 g/(kg · d), 95% CI: 0.14–0.36 g/(kg · d)].
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The comparison between means (breakpoint or requirement) resulted in a significant difference (P < 0.005). This difference indicates that when excess cysteine is provided, the intravenous methionine requirement is 72% of the enteral requirement. Further comparison with our previous study (7) in which we measured the methionine requirement without cysteine resulted in significant differences within route of feeding (P < 0.05, Table 1). These differences indicate that cysteine can spare the dietary methionine requirement by 40% in both enteral and parenteral feeding.
Effects of cysteine supplementation on whole-body protein synthesis.
This was determined by comparing the data in the current study with the data from our recently reported work (7), conducted under identical experimental conditions, in which piglets were fed methionine only. We compared the percentage of the phenylalanine dose oxidized at plateau among the four dietary treatments. Plateau oxidation (as a % of dose) did not differ among dietary treatments (P > 0.05); therefore, whole-body protein synthesis was not different with or without dietary cysteine. We can therefore conclude that dietary cysteine is not a dietary indispensable amino acid in neonatal piglets.
Plasma cysteine.
Dietary methionine intake was positively correlated with total plasma cysteine concentrations in enterally fed piglets (i.g.-CM) (P < 0.05; Fig. 2), but was not correlated in the parenterally fed piglets (i.v.-CM) (P > 0.05; Fig. 2). We also measured the total plasma cysteine in piglets that were administered stepwise methionine intakes during both parenteral and enteral feeding with no dietary cysteine (7). These new data are included in the present paper for a more complete comparison of the effects of methionine intake and route of feeding on plasma total cysteine concentrations (Fig. 2). When piglets were fed methionine without dietary cysteine (7), dietary methionine intake was positively correlated with total plasma cysteine during enteral (i.g.-M) but not parenteral (i.v.-M) feeding. Plasma cysteine was significantly different among all treatments; it was highest in the i.v.-CM group (334 µmol/L), which was greater than the i.g.-CM group (139 µmol/L). Both groups fed excess cysteine had greater plasma cysteine concentrations than the groups fed methionine alone; the i.g.-M group had higher concentrations (65 µmol/L) than the i.v.-M group(38 µmol/L) (P < 0.01).
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The mean parenteral methionine requirement when excess cysteine was provided, as determined by a two-phase linear crossover model, was 0.18 g/(kg · d). The mean enteral requirement was 0.25 g/(kg · d). The upper 95% CI was estimated and is assumed to meet the methionine requirement of 95% of the population. This safe level of intake of methionine was determined to be 0.27 and 0.36 g/(kg · d) in parenteral and enteral feeding, respectively. The significant difference between breakpoints suggests that the gut is using 28% of the TSAA requirement (Table 1). This closely matches our previous estimate of splanchnic extraction (7) of 31% of the methionine intake (Table 1).
When measuring the amount of cysteine required to optimize performance, one must account for the differences in molecular weight between methionine and cysteine. Because of this molecular weight difference, the efficiency of methionine in meeting the biological need for cysteine on a weight or concentration basis is on the order of 80%, whereas an assumption of 100% is frequently (57) and incorrectly used. Therefore, our present estimate of 40% of the methionine requirement spared on a weight basis equates to cysteine comprising 50% of the TSAA requirement.
In growing pigs, the maximal proportion of the sulfur amino acid requirement that can be met by cysteine has been estimated to range between 40 and 70% (9–15). Presently, the NRC (49) suggests that methionine and cysteine be supplied in a 50:50 ratio (wt:wt) for swine of all ages. However, few data exist on pigs < 3 kg; therefore, the majority of recommendations for these piglets are extrapolated from experiments that determined the requirements of older pigs. Our estimate of the enteral cysteine sparing capacity (40% in both routes of feeding) is comparable to previous estimates in growing pigs. Kim and colleagues (15) used the IAAO method and oral isotope dosing to measure the methionine requirements in piglets of a similar age (10–14 d old) and weight (3 kg) as those in the present study. The piglets were fed a diet based on free amino acids and dried skim milk. The methionine requirement, when excess cysteine and choline were provided, was 2.7 g/kg diet. This calculates into a requirement of 0.26 g methionine/(kg · d). Within the same study, they measured the methionine requirement to be 0.43 g/(kg · d) when no dietary cysteine was provided. This is remarkably similar to our estimates of 0.42 g/(kg · d) for methionine alone and 0.25 g/(kg · d) when cysteine is supplied in excess. Chung and Baker (57) measured the maximal proportion of the dietary sulfur amino acids that could be supplied by cysteine in 10-kg pigs fed a chemically defined amino acid diet. Methionine:cystine ratios of 60:40 and 50:50 (wt:wt) supported greater weight gain than those observed in pigs fed 40:60. The 50:50 ratio is comparable to our estimate, resulting in cysteine meeting 40% of the TSAA requirement on a methionine weight basis. Given the agreement between these results, it is apparent that cysteine cannot supply >50% of the TSAA requirement. Conversely, cysteine can replace 40% of the methionine requirement if methionine were meeting 100% of the TSAA requirement. Our data demonstrate for the first time that this recommendation also applies to parenteral feeding, for which the estimate as a proportion of the methionine requirement was not different.
The cysteine sparing effect on the methionine requirement that we observed in the present study is supported by in vitro data that demonstrated a correlation between increases in dietary cysteine and the concomitant reduction in the activity of the CBS enzyme in the liver. Supplemental dietary cysteine significantly reduced hepatic CBS activity (29–31). Increasing dietary cysteine up to 50% of the sulfur amino acids decreased CBS activity but did not affect growth (28). This reduction in CBS activity reduces the rate of transsulfuration and subsequently decreases the endogenous synthesis of cysteine. Therefore, there is a reduction in transsulfuration and the dietary methionine requirement is reduced when dietary cysteine is increased. In the present study, the similarity in sparing capacities between routes of feeding suggests that the rate of transsulfuration was not different when first-pass metabolism by the small intestine is bypassed.
Cysteine was reported to be conditionally essential for premature neonates due to the low activity of the transsulfuration enzyme cystathionase in fetal tissue and a greater concentration of its precursor cystathionine (2,4,58). In addition, other research has demonstrated that plasma cyst(e)ine concentrations are dramatically decreased in neonates administered TPN (59). However, cystathionase activity is positively correlated with the gestational and postnatal age of the infant (60), suggesting that term infants may not have limited cysteine synthesis. In addition, cystathionase activity is present in both the adrenals and kidneys of both premature and term infants. In vitro experiments indicate that premature infants are potentially capable of synthesizing adequate endogenous cysteine if provided with adequate dietary methionine (60). Furthermore, Zlotkin et al. (61) reported that cysteine supplementation to TPN did not improve nitrogen retention in term or premature infants but it did normalize plasma concentrations of cysteine. In our studies, there was no evidence to support the claim that cysteine is indispensable in neonates; however, we did observe the lowest plasma cysteine concentrations in TPN-fed piglets fed no cysteine (7) (Fig. 2). The oxidation of the indicator amino acid (phenylalanine), at adequate methionine concentrations, was similar in our two studies with and without dietary cysteine (Table 1). This suggests that there is not a separate requirement for cysteine. A lower baseline oxidation when cysteine was provided would have indicated an essential requirement for cysteine that could not be met by methionine. However, if cysteine use for protein synthesis has priority over its use for the synthesis of products, such as taurine and glutathione, then we may not have measured the total requirement for methionine for both protein synthesis and conversion to specific products.
Plasma total cysteine concentrations were profoundly different among all treatment groups (Fig. 2). The i.g.-M treatment had significantly higher plasma cysteine concentrations than the i.v.-M treatment, indicating that either the gut produces cysteine and/or parenterally fed piglets require more cysteine for glutathione, taurine or sulfate synthesis. A positive correlation between dietary methionine intake and plasma cysteine concentrations in the enterally, but not the parenterally, fed treatments, further suggests that the gut is involved in cysteine synthesis. In addition, we observed the lowest concentrations in piglets administered the i.v.-M treatment. However, piglets administered the i.v.-CM treatment had higher plasma cysteine concentrations than those given the i.g.-CM treatment, suggesting that the gut was responsible for a reduction in the synthesis and release of cysteine into the systemic circulation when cysteine was given enterally. All of the enzymes for the transulfuration pathway are present in the gut (1); therefore, the presence of enterally fed cysteine may spare the catabolism of methionine via the transulfuration pathway (via negative inhibition of CBS), whereas the cysteine released from the gut increased incrementally with increased concentrations of the dietary SAA. The low plasma cysteine concentrations found in the i.v.-M treatment agree with previous studies that have found low plasma cysteine concentrations during parenteral feeding in neonates (5,62), young rabbits (8) and adults (6). Infants who received TPN supplemented with cysteine had 60% greater plasma cysteine than an unsupplemented group, even though nitrogen balance did not change (61). Furthermore, in piglets administered the i.g.-CM treatment, plasma cysteine concentrations were consistent with dietary concentrations of cysteine, similar to results obtained in rats fed a diet that supplied both methionine and cysteine (63). The liver plays the dominant role in cysteine catabolism (64); thus, it is unlikely that the difference found between pigs administered the i.g.-CM and i.v.-CM treatments was due to extensive cysteine oxidation in the gut. However, cysteine is metabolized to some extent by all tissues, including the small intestine; therefore, the gut may have been partly responsible for cysteine catabolism. Clearly, the gut is involved in the metabolic regulation of the sulfur amino acids; it likely synthesizes cysteine when the diet is lacking in cysteine and reduces its synthesis from methionine when the diet contains adequate amounts of cysteine.
In the present study, the concentration of many plasma amino acids decreased as methionine intake increased (Tables 6, and 7). We previously reported a similar response to dietary BCAA (41). This change in plasma amino acids may represent the increasing incorporation of these amino acids into protein with increasing intake of the limiting amino acid. In addition, at low methionine intakes, we observed significant increases in the plasma concentrations of glutamate and glutamine, indicating that there was an increased need to carry nitrogen to the urea cycle for disposal. Taurine, a beta amino acid that is not incorporated into protein, decreased as methionine intake increased in both parenteral and enteral feeding (Tables 6, and 7), reached it lowest concentrations at the diet level closest to the estimated breakpoint [0.2 and 0.25 g/(kg · d), respectively] and then increased at the highest diet levels. The excess cysteine at low and high methionine intakes may have been shunted toward taurine synthesis. This implies that as dietary methionine was increased, the incorporation of cysteine into protein increased; thus the synthesis of taurine decreased until the concentration of methionine reached requirement, and then taurine concentrations rose once again. Indeed, hepatic cysteine dioxygenase activity and urinary excretion of taurine were elevated in rats fed diets with excess cysteine (65). Premature infants may have low or absent cysteinesulfinate decarboxylase (EC 4.1.1.29) activity and increased renal taurine losses (66). However, the differences in plasma taurine concentrations that we observed among diet treatments suggests that cysteinesulfinic acid decarboxylase activity is not limited in neonatal piglets administered either enteral or parenteral nutrition. Furthermore, we observed much higher concentrations of plasma taurine than in our previous studies (7,41,67). This supports the interpretation that the excess supply of cysteine concomitantly increased the synthesis of taurine.
In conclusion, dietary cysteine can spare the methionine requirement for optimal protein synthesis by 40% in both parenterally and enterally fed piglets. A comparison of the current data with piglets fed methionine alone showed that cysteine is not a dietary essential amino acid in neonatal piglets. Furthermore, in the presence of excess cysteine, the parenteral requirement for methionine is 30% lower than the enteral requirement and this difference due to route of feeding is similar to our previous estimate when no cysteine was provided (7).
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FOOTNOTES |
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2 Supported by grants from Alberta Pork, The Alberta Agricultural Research Institute, Degussa, AG, by the CIHR Fund # 12928 and NSERC (J.D.H.).
4 Abbreviations used: CBS, cystathionine ß synthase, IAAO, indicator amino acid oxidation, SAA, sulfur amino acids, SRA, specific radioactivity, TPN, total parenteral nutrition, TSAA, total sulfur amino acids.
Manuscript received 9 July 2003. Initial review completed 11 August 2003. Revision accepted 13 September 2003.
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LITERATURE CITED |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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