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Department of Animal and Poultry Science, University of Guelph, Guelph, ON, Canada N1G 2W1;
* U.S. Department of Agriculture/ARS Children’s Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, TX; and
Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, AB, Canada T6G 2P5
3To whom correspondence should be addressed. E-mail: dburrin{at}bcm.tmc.edu.
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ABSTRACT |
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 TOP ABSTRACT LITERATURE CITED |
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KEY WORDS: • methionine • cysteine • homocysteine • transmethylation • transsulfuration • neonate
There is increasing evidence that sulfur amino acids (SAAs),4 methionine and cysteine, play an important metabolic and functional role in human health and disease. A greater understanding of whole-body metabolism of the SAAs is needed. Clinical studies indicate that elevated plasma levels of homocysteine, a key product of methionine metabolism, are strongly associated with increased risk of cardiovascular disease and more recently, Alzheimer’s disease (1,2) in adults, and with ischemic and hemorrhagic stroke in infants and children (3,4). Evidence suggests that the prooxidant properties of homocysteine contribute to oxidant stress and vascular injury, although the causal link between homocysteine and these diseases has not yet been established. Cysteine also plays a key role in cellular redox status by virtue of its thiol (–SH) moiety, but also as a precursor of glutathione, a major cellular antioxidant. The liver is generally considered to be the primary site of dietary methionine and cysteine metabolism in the body. However, in this review, we will discuss the evidence suggesting that intestinal sulfur acid metabolism may be nutritionally relevant and why it is important for gut function.
SAA metabolism.
Methionine is a dietary indispensable amino acid because it cannot be synthesized in amounts sufficient to sustain normal growth in mammals. However, many tissues of the body are capable of converting methionine to cysteine via the enzymatic processes of transmethylation and transsulfuration (5). Because a portion of dietary methionine is normally converted to cysteine, numerous studies have shown that providing dietary cysteine can "spare," reduce, or replace a portion of the requirement for methionine by as much as 50–80% in mammals and birds. Three important metabolic functions of methionine are: 1) transmethylation to form a primary methyl donor, S-adenosylmethionine (SAM), which methylates compounds to form products such as creatine and phosphatidylcholine, resulting in the product of methylation homocysteine being produced (Fig. 1); SAM can also be decarboxylated to form decarboxylated SAM, which then donates an aminopropyl group for polyamine synthesis after which methionine will be reproduced; 2) transsulfuration to form cysteine, which in turn is catabolized to taurine or incorporated into glutathione; and 3) protein synthesis. Methionine can also enter the body pool by remethylation of homocysteine or protein breakdown. Key enzymes in the transsulfuration of methionine are: 1) cystathionine ß-synthase, which condenses serine and homocysteine to form cystathionine, and 2) 
-cystathionase, which then cleaves cystathionine into cysteine and 
-ketobutyrate.
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Nutritional consequences of intestinal essential amino acid utilization. Previous studies in infant piglets showed that the net portal absorption of several essential amino acids, including methionine and BCAAs, is considerably less than 100% of the dietary intake, ranging from 40 to 70% (14). In addition, these studies suggest that as much as 30–60% of the dietary intake of leucine, lysine, and threonine is sequestered in first-pass utilization by the gut and metabolized. Net intestinal utilization of methionine was substantial, consuming 52% of the dietary intake. Subsequent in vivo studies with 13C-labeled lysine and leucine showed that the gut represents a substantial proportion (20–30%) of whole-body oxidation of essential amino acids (15,16). The importance of the gut was also shown in piglet studies designed to estimate the whole-body amino acid requirements by indicator amino acid oxidation in enterally and parenterally fed neonatal piglets (11,17–19). These studies indicated that the whole-body requirements for threonine, BCAAs, and methionine were significantly higher in enterally fed than in parenterally fed piglets. The methionine requirement was 30% greater in enterally fed than parenterally fed piglets fed methionine alone or in combination with excess cysteine (11,19). These data suggest that intestinal metabolism of dietary methionine is nutritionally relevant.
Evidence of intestinal SAA metabolism. The original studies by Finkelstein (5) demonstrated that gastrointestinal tissues possess the enzymes necessary to metabolize methionine to cysteine, albeit at lower levels of activity than the liver. However, there are few reports describing the kinetics of methionine metabolism in the gut, either in vivo or in vitro with isolated enterocytes. Recent studies based on in vivo isotopic tracers in ruminants imply that methionine transmethylation occurs in the ruminant gut (20). Methionine transmethylation in the gut, without subsequent remethylation or transsulfuration, could result in net release of homocysteine into the circulation. Indeed, recent in vitro studies demonstrated that Caco-2 cells, a model of human colonic epithelial cells, produced substantial amounts of homocysteine and cystathionine, suggesting that the intestine may export homocysteine and contribute to the plasma homocysteine load (21). In agreement with these in vitro data are in vivo data that showed plasma total homocysteine to be significantly higher in enterally fed piglets than in parenterally fed piglets fed a similar diet, without any cysteine (22).
In the case of dietary cysteine, studies in pigs indicate that the rate of appearance in the portal blood is very limited (<20% dietary intake), suggesting extensive intestinal utilization of cysteine (14,23). The first step in cysteine catabolism is conversion to cysteine sulfinate via the enzyme cysteine dioxygenase. Approximately 70–90% of cysteine sulfinate is subsequently decarboxylated via cysteine sulfinate decarboxylase to produce hypotaurine, which is then oxidized to taurine via a poorly characterized enzymatic reaction (24). Alternatively, cysteine sulfinate may undergo transamination or oxidative deamination to form the putative intermediate, ß-sulfinylpyruvate, which spontaneously decomposes to yield pyruvate and sulfite; this pathway accounts for 10–30% of cysteinesulfinate degradation. Rodent studies with 14C-labeled cysteine demonstrated significantly higher oxidation when given via the intragastric (70%) than intraperitoneal (41%) route, suggesting that nearly half of the whole-body cysteine oxidation occurs in splanchnic tissues (24). More importantly, subsequent work demonstrated that intestinal enterocytes extensively metabolize cysteine via cysteine dioxygenase to cysteinesulfinate (25). In vivo rodent studies with i.v. infusion of isotopically labeled 15N-cysteine indicated that an important metabolic fate of cysteine in the gut is incorporation into glutathione (GSH), which would not involve oxidation of cysteine (26). Thus, it appears that the intestine has the enzymatic capacity for transmethylation and transsulfuration of methionine, and for oxidation of cysteine and GSH synthesis.
Importance of SAAs in total parenteral nutrition (TPN)-fed neonates. The metabolism of SAAs is particularly important in the nutritional support of neonatal infants. In neonatal animals, the slow maturation of the enzyme cystathionase may limit de novo cysteine synthesis (27–29). However, cystathionase activity is present in the adrenals and kidneys of both premature and term infants, suggesting that term infants may not require additional cysteine (30). A second key consideration is that most preterm infants are administered TPN for a period of days to weeks before full enteral feeding is attained. Studies in parenterally fed, preterm infants (29 wk gestational age) showed that cysteine synthesis is virtually absent (31). Furthermore, in human neonates and adults and in piglets, parenteral feeding caused a reduction in plasma cysteine and cystine (9–11). These findings led to the idea that cysteine is conditionally indispensable in parenterally fed neonates, yet few commercial parenteral solutions contain appreciable amounts of cysteine and others contain no cysteine at all. Furthermore, if the gut is an important site of transsulfuration, then the observation of low plasma cysteine concentrations under conditions of TPN may be a result of the bypassing of intestinal methionine metabolism.
Oxidant stress and intestinal SAA metabolism. SAAs, especially cysteine, play a key role in antioxidant status and cellular function (32,33). Glutathione is the most important cellular antioxidant in mammals and has a critical function in responding to reactive oxygen species and maintaining cellular redox status. Reduced glutathione (GSH) is a ubiquitous tripeptide (-Glu-Cys-Gly) present throughout the body at relatively high intracellular concentrations, especially in the small intestine. Cellular GSH homeostasis is maintained through de novo synthesis from precursor SAAs (methionine and cysteine), regeneration from its oxidized form glutathione disulfide (GSSG), and uptake of extracellular intact GSH. Mediating oxidant stress and maintaining normal redox status is especially important in intestinal epithelial cells, which function as an innate defense barrier against luminal toxins and oxidants derived from the diet. In this regard, glutathione is essential for normal intestinal function (34) and is related to an increased susceptibility to carcinogenesis, oxidative injury, metal intoxication, and common intestinal pathologies (35). Studies with intestinal epithelial cells indicate that increased oxidant stress and redox imbalance suppress cell proliferation and induce apoptosis and that this is closely correlated with a higher oxidized glutathione state, as measured by the ratio of GSH:GSSG (36–38). In vitro studies found that cells grown in cysteine-deficient media have suppressed GSH concentrations and cell proliferation rates, both of which are increased with cysteine supplementation (39,40). Other studies with human colonic epithelial cells (Caco-2) indicated that as differentiation proceeds, cell GSH concentration and proliferation rate decrease, whereas the apoptosis rate increases (41). Collectively, these studies suggest that cysteine availability and local GSH concentration have a direct influence on epithelial cell proliferation and survival and is inversely proportional to cellular differentiation state.
Methionine transsulfuration and intestinal cell function. A possible mechanistic link between methionine transsulfuration and intestinal function is the role of methionine in epithelial cell turnover and antioxidant status. It is evident from the foregoing discussion that cysteine availability is important for maintenance of epithelial cell GSH level and cell survival. However, it is poorly understood whether methionine can affect cysteine availability in epithelial cells via transsulfuration, and hence cell function. Evidence in support of this idea is the finding that in HepG2 cells, oxidant stress increased transsulfuration measured by cystathionine synthesis and 35S-methionine incorporation into glutathione (42). In addition, homocysteine flux through the transsulfuration pathway is decreased when exposed to antioxidants (43). Moreover, studies in HepG2 cells also showed that cystathionine synthase activity is coordinately regulated with proliferation via a redox-sensitive mechanism (44). These results imply that cells exposed to oxidant stress may meet the increased cysteine requirement for GSH synthesis via activation of methionine transsulfuration. A broader implication of this result is that maintenance of normal intestinal epithelial cell proliferation and survival requires active methionine transsulfuration for synthesis of cysteine and GSH.
In conclusion, there is substantial evidence that first-pass splanchnic metabolism, specifically by the intestine, plays an important role in SAA metabolism. Transmethylation, remethylation, transsulfuration, and glutathione synthesis were shown to be affected by the addition or removal of nutrients, methionine and cysteine, enzymatic cofactors, and antioxidants and that this metabolism is regulated differently when intestinal metabolism is bypassed during parenteral feeding. The gut may be a quantitatively important site for conversion of dietary methionine to both homocysteine and cysteine, yet there is limited direct evidence to confirm this idea. If this were confirmed, it may have implications for nutritional strategies to manipulate homocysteine metabolism as a means to reduce the risk of cardiovascular disease and stroke. Given that first-pass splanchnic metabolism plays an important role in SAA metabolism, there are also implications for the formulation of parenteral solutions. The functional importance of dietary methionine and cysteine for intestinal growth and function, beyond its role as a precursor for protein synthesis, warrants further investigation. Furthermore, given that cysteine has a key role in cellular antioxidant function, which is a determinant of cell proliferation and survival, understanding to what extent methionine can serve as an intracellular precursor for cysteine in intestinal epithelial cells also warrants further study.
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FOOTNOTES |
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2 Portions of the work cited in the review were supported by federal funds from the U.S. Department of Agriculture, Agricultural Research Service under Cooperative Agreement NO 58–6250-6–001, the National Institutes of Health grant HD33920 (D.G.B.).
4 Abbreviations used: GSH, glutathione; GSSG, glutathione disulfide; SAA, sulfur amino acid; SAM, S-adenosyl-methionine; TPN, total parenteral nutrition.
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LITERATURE CITED |
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TOPABSTRACTLITERATURE CITED |
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