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USDA/ARS Children’s Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, TX 77030
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 Top Abstract INTRODUCTIONTERMINOLOGYMETABOLIC FATE OF AMINO...SUMMARY AND FUTURE PERSPECTIVESLITERATURE CITED |
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Key Words: intestine • nutrition • swine
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INTRODUCTION |
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TopAbstractINTRODUCTIONTERMINOLOGYMETABOLIC FATE OF AMINO...SUMMARY AND FUTURE PERSPECTIVESLITERATURE CITED |
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TERMINOLOGY |
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TopAbstractINTRODUCTIONTERMINOLOGYMETABOLIC FATE OF AMINO...SUMMARY AND FUTURE PERSPECTIVESLITERATURE CITED |
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). Whole-body tracer kinetic models assume that the amount of AA fed in the diet is the amount of AA that is entering the metabolic pool from outside the body (Figure 1). First-pass extraction by splanchnic tissues describes the proportion of ingested AA that is sequestered during its initial transit through the splanchnic bed and thus not appearing in systemic blood. Once extracted, individual AA can be utilized (in first pass); that is, undergo different metabolic fates. The fraction of a dietary AA taken up (sequestered) during the first pass through splanchnic tissues after digestion and absorption can be estimated by simultaneously administering different isotopomers of a stable isotope-labeled AA intravenously (e.g., 2H-leucine) and intragastrically (e.g., 13C-leucine) (Hoerr et al., 1991). Any removal of the intragastric tracer on the first pass through the gut (i.e., PDV) and liver will be reflected by a lower (systemic) plasma enrichment of the intra-gastric tracer compared with that achieved with the intravenous tracer.
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This method quantifies the net amount of nutrients absorbed into the portal circulation. However, the net portal balance estimate does not distinguish between dietary nutrients used by the intestine in first-pass and systemic nutrients extracted from the arterial circulation by the PDV. This can be accomplished by using intravenous and intragastric tracer AA and measuring the isotopic enrichment in portal (IEPORT) and arterial (IEART) blood. By calculating the tracer balance, the intestinal (PDV) use of dietary (i.e., intragastric tracer) and systemic (i.e., intravenous tracer) AA can be quantified:
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The term tracer balance is sometimes used to represent a unidirectional process, either absorption or use by the tissue, to distinguish whether the substrate is derived from the diet or arterial circulation. Thus, unidirectional intestinal uptake of an intragastric tracer AA refers to the uptake of a dietary substrate across the apical membrane of mucosal epithelial cells. Hepatic extraction is defined by the difference between splanchnic extraction and portal extraction.
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METABOLIC FATE OF AMINO ACIDS |
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TopAbstractINTRODUCTIONTERMINOLOGYMETABOLIC FATE OF AMINO...SUMMARY AND FUTURE PERSPECTIVESLITERATURE CITED |
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; Wolff et al., 1972; Felig, 1975). More detailed kinetic information of unidirectional AA fluxes was subsequently obtained by adapting this approach to the use of radioactive, isotopic AA tracers (Heitmann and Bergman, 1978). More recently, the use of combined measurements of the net splanchnic organ and portal balance together with enteral or intravenous infusions of stable isotope-labeled AA has provided an in vivo model that is capable of assessing the splanchnic and intestinal use of nutrients derived from the diet and systemic circulation in humans, pigs, dogs, mice, sheep, and cattle (Hoerr et al., 1991; Yu et al., 1995; Lobley et al., 1996b; Stoll et al., 1998; Lapierre et al., 1999; Hallemeesch et al., 2001).
The metabolic fate of AA in the splanchnic tissues differs not only between the liver and gastrointestinal tissues, but also among cells within each tissue bed. The latter is significantly affected by how AA are presented to the tissue. It is important to recognize that within the PDV, AA are presented via both the lumen and the arterial circulation. From a quantitative perspective, the supply of most AA from the arterial circulation is substantially greater (3- to 5-fold) than that from the diet (Figure 2). However, the fractional PDV use (i.e., uptake/intake ratio) of dietary AA, ranging from 95 to 20%, is generally much greater than AA derived from the arterial blood, ranging from approximately 5 to 15%. The reason for this observation is unclear, but may be due to differences in AA transporter abundance on the basolateral and apical surfaces of intestinal epithelial cells or the low residence time of the substrate near the site of the transporter on the basolateral surface.
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). Within a tissue, the morphological localization of a cell can dictate the metabolic fate of AA. For example, in the gut, the extent to which epithelial cells derive their AA from the luminal or vascular input is affected by their stage of differentiation and physical location along the crypt–villus axis. Studies showed that crypt cells are more highly labeled with isotopic tracers given intravenously (Alpers, 1972). The results implied that crypt cells derive nutrients predominantly from the arterial circulation, whereas villus cells rely on nutrients absorbed luminally from the diet. Likewise, in the liver, studies have shown a cellular zonation in the acinus, in which periportal hepatocytes scavenge excess portal ammonia by ureagenesis, whereas perivenous hepatocytes sequester ammonia via glutamine synthesis (Haussinger, 1990). Once taken up by the splanchnic tissues, AA have 3 possible metabolic fates: 1) incorporation into protein; 2) conversion into other AA or use for other biosynthetic purposes; and 3) complete oxidation to CO2. In the case of intestinal epithelial cells, another possibility is that once AA are transported into the cell they can then be transported out of the cell and into the portal blood stream. In the first 2 pathways, AA can be deposited and recycled by the body for purposes of growth or other biological functions. This recycling process may be affected by whether an AA is incorporated into a constitutive protein or secreted protein. However, from a nutritional perspective, if essential AA are irreversibly metabolized or completely oxidized to CO2, this represents a nutritional loss to the animal.
If we first consider the metabolic fate of dietary AA in the gut, there are some general observations that can be made from estimates of the net portal balance expressed as a proportion of intake (Table 1). It is also important to note that the mode of enteral feeding in all 4 studies described in Table 1 varied considerably, between bolus vs. continuous and gastric vs. duodenal administration of the exact same diet. Many of the values are less than 100%, although it has been shown that the digestibility of many common dietary proteins, in this case milk protein, approaches 100% (Fan et al., 1994). The difference between digestibility and portal appearance could be due to the absorption of dietary protein in the form of small peptides into the portal blood, as has been suggested for calves, steers, sheep, and rats (Seal and Parker, 1991; Koeln et al., 1993). However, studies in milk-fed piglets infused intragastrically with 13C-labeled algae protein did not show any evidence of 13C-labeled peptides in the portal blood (B. Stoll and P. J. Reeds, CNRC/Baylor College of Medicine, Houston, TX; unpublished data). The first observation from Table 1 is that the net balances of glutamate, glutamine, and aspartate are nearly zero. In other words, the net use of these AA by the PDV is approximately equal to the dietary intake. In some cases, the net balance of glutamine is negative even in the fed state due to the high rate of metabolism in gastrointestinal tissues. As will be discussed below, the high fractional metabolism of these AA is due to their integral role as oxidative fuel. A second remarkable observation is that the net balance of arginine, alanine, and in some cases tyrosine and proline, is greater than 100%, which suggests a net production of these AA by the gut tissues. The third observation is that the net balance of many essential AA is significantly less than 100%, and in some cases less than 50% of the dietary intake.
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). Yet, very little of the threonine utilized by the gut is oxidized to CO2; thus, presumably much of this threonine is recycled back into the portal blood for use in the postabsorptive period (van der Schoor et al., 2002). In the case of lysine, gut use accounts for 50% of the dietary lysine intake and represents approximately 30% of whole-body oxidation. However, gut lysine oxidation only accounted for 5% of the dietary intake; thus, the true impact of gut essential AA metabolism on peripheral growth depends mainly on the extent of oxidation. The biological explanation for the differences in the proportion of dietary AA utilized by the gut is only partially understood at present and requires further study to specifically establish the functional purpose of each AA.
In contrast to the PDV, there are few reports in the literature describing the net AA balance by the liver in nonruminants, particularly in the fed state (Elwyn et al., 1968; Barrett et al., 1986; Rerat et al., 1992; de Blaauw et al., 1996). There are numerous reports of hepatic AA balance in ruminants (Wolff et al., 1972; Lobley et al., 1996b; Lapierre et al., 1999; Blouin et al., 2002). A study in pigs infused enterally for 1 h (Rerat et al., 1992) demonstrated that, within 8 h, the net hepatic uptake of glycine and alanine is substantially greater (150 to 250%) than the dietary intake. The only AA significantly released during that period were glutamate and aspartate; the net balance of glutamine was essentially zero. Among the remaining AA, the cumulative hepatic uptake increased from 15 to 50% of the dietary intake between 1 and 8 h, perhaps indicative of postabsorptive release of AA that had initially been extracted from the diet by intestinal tissues in first-pass. Interestingly, the cumulative net uptake of the branched-chain AA was lower (35 to 43% of intake) than the other essential AA; this later observation translates into greater splanchnic output of branched-chain AA compared with other essential AA.
In addition to the animals in which gut and liver metabolism have been measured separately, there have been numerous reports of total splanchnic AA uptake in adult humans using stable isotopes (Castillo et al., 1993a; Matthews et al., 1993; Battezzati et al., 1995; Haisch et al., 2000). These studies demonstrated the substantial first-pass splanchnic extraction (percentage enteral input) of AA, including glutamate (96%), glutamine (64%), alanine (69%), arginine (38%), leucine (21%), and phenylalanine (29%). A recent report in humans describing the kinetics of ingested 15N-labeled soy protein showed that the splanchnic bed extracted nearly 60% of the dietary N, of which 40% was channeled into protein synthesis and 20% was deaminated (Fouillet et al., 2003). Similar studies examining leucine kinetics found that first-pass splanchnic uptake in preterm infants and elderly men were approximately 2-fold greater than in young adult men (Beaufrere et al., 1992; Boirie et al., 1997).
Protein Synthesis and Secretion
A major metabolic fate of AA taken up by the gut and liver is cellular protein synthesis. In comparison to splanchnic tissues (mainly small intestine and liver), the protein synthesis rates in other visceral organs and peripheral tissues are substantially lower. Notably, skeletal muscle generally has the lowest protein synthesis rate among all tissues, yet comprises the largest proportion of whole-body protein mass.
Amino acids derived from both luminal and arterial sources can be incorporated into constitutive proteins (Alpers, 1972; Bouteloup-Demange et al., 1998; Stoll et al., 1999b). Given the very high rates of protein synthesis in the intestine, only about one-third of the intestinal first-pass metabolism of dietary lysine, leucine, and phenylalanine is incorporated into mucosal protein (Stoll et al., 1998). Results from studies in fed piglets with intravenous or intragastric tracer AA suggest that there is kinetic channeling of arterial AA toward mucosal protein synthesis (Dudley et al., 1994; Bouteloup-Demange et al., 1998; Dudley et al., 1998; Stoll et al., 1999b).
With threonine, another aspect contributing to the overall protein synthetic activity in the intestine comes into account. Unlike in most organs, intestinal protein synthesis is not only devoted to the renewal and accretion of cellular protein, but also to proteins that are secreted into the lumen. Ultimately, the net protein loss that affects AA requirements depends on the balance between the rate of secretion and the degree to which secreted proteins are recycled or irreversibly lost.
Approximately 60% of dietary threonine is utilized by the gut; only about 10% of this is recovered in mucosal protein (Stoll et al., 1998). It has been speculated that the majority of utilized dietary threonine is incorporated into threonine-rich mucins (Roberton et al., 1991; Bertolo et al., 1998). Under conditions of total parenteral nutrition, in which intestinal growth, protein synthesis (Burrin et al., 2000; Stoll et al., 2000), and therefore, possibly mucin production are compromised, threonine requirement was <50% of that in piglets receiving enteral nutrition (Bertolo et al., 1998). Hence, intestinal protein secretion appears to substantially affect the whole-body threonine needs.
Amino Acids as Oxidative Fuels
The tissues of the splanchnic bed derive a majority of their oxidative energy from the catabolism of AA, rather than glucose or fatty acids. Historically, the liver has been considered a major site of AA catabolism and oxidation. However, since the classic studies of Windmueller and Spaeth (Windmueller and Spaeth, 1974, 1975, 1976, 1978, 1980), it has become apparent that the intestine also is a major site of catabolism of AA. Nevertheless, the extent to which AA are completely oxidized to CO2 may vary within each tissue. In the liver, the primary metabolic fate of dietary AA carbon is its conversion to glucose (Jungas et al., 1992), which is one way to make energy available to peripheral tissues. The gut tissues are releasing carbon derived from nonessential AA catabolism into the portal vein as alanine and lactate, both of which are key precursors for hepatic gluconeogenesis (Brosnan, 2000, 2003). Thus, it is not surprising that the PDV release proline, arginine, and ornithine into the portal vein—AA for which metabolism converges at glutamate in the liver. Hence, it becomes apparent that the splanchnic tissues are situated anatomically and metabolically within the mammalian organism to regulate the flow of dietary AA in such manner as to meet their oxidative energy needs first, at the same time ensuring delivery of the primal oxidative fuel for peripheral tissues, namely glucose.
The seminal studies of Windmueller and Spaeth (Windmueller and Spaeth, 1974, 1975, 1976, 1978, 1980) were the first to show evidence of extensive metabolism of glutamine, glutamate, and aspartate in in situ intestinal perfusions in fasted, anesthetized rats. Results from young piglets fed a high-protein, milk-based formula indicated that more than 95% of the dietary glutamine, glutamate, and aspartate is utilized by the gut tissues (Stoll et al., 1998). The studies of Windmueller and Spaeth focused attention on the role of glutamine as the major oxidative fuel in the gut. However, it is important to note that both glutamate and aspartate are of perhaps equal importance as intestinal oxidative fuels. Recent studies in young pigs and humans confirm the extensive intestinal oxidation of dietary 13C-labeled glutamate (Figure 3) and glutamine (Battezzati et al., 1995; Stoll et al., 1999a; Haisch et al., 2000).
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, 1978; Stoll et al., 1999a). The remaining carbon atoms from these 3 substrates, which are not oxidized to CO2, are converted to lactate, alanine, proline, citrulline, ornithine, and arginine, and then released into the portal circulation (Figure 3
; Windmueller and Spaeth, 1975; Stoll et al., 1999a). The metabolic fate of nitrogen from these AA is not fully understood. Yet, evidence suggests that a portion of the nitrogen derived from glutamine and glutamate metabolism is released as ammonia or transferred to other AA, including citrulline, ornithine, proline, and arginine, a major portion of which is converted to urea in the liver (Reeds et al., 2000).
Interestingly, in the fed state, only 6% each of dietary and systemic glucose intake is utilized by the intestinal tissues, whereas 2 and 27% of each fraction, respectively, is converted to CO2. However, because of its much greater arterial supply in absolute terms (5-fold), systemic glucose contributes 29% to total CO2 production by the PDV (Stoll et al., 1999a). Despite its importance as oxidative fuel, the proportion of glucose oxidized completely to CO2 is substantially less than that of either glutamine or glutamate, which implies that most of the glucose is utilized for other metabolic or biosynthetic purposes (Stoll et al., 1999a). This pattern of substrate use for energy generation changes when the supply is limited (van der Schoor et al., 2001). In vivo studies in piglets with stable isotope-labeled glucose and glutamate demonstrated that, during protein restriction (40% of control) but isocaloric feeding conditions, intestinal CO2 production from dietary glutamate decreases whereas intestinal tissues adapt by oxidizing dietary glucose and simultaneously lowering the oxidation of systemic glucose (Figure 4; van der Schoor et al., 2001). Similarly, the intestinal oxidation of dietary and systemic leucine was suppressed in protein-restricted animals, and only 60% (vs. 89% during normal protein intake) of the CO2 production by the PDV could be accounted for, indicating that perhaps lipids are used for PDV CO2 production. In contrast, during excessive glutamate intake, the fractional splanchnic use was 97 and 88% of intake at 100 and 300% dietary glutamate load, respectively (Stoll et al., 2005). Even under these conditions, glutamate continued to be metabolized preferentially by intestinal tissues rather than the liver. The fractional intestinal CO2 production from enteral glutamate was 49 and 33% at normal and high glutamate intake levels, respectively. These results suggest that, although dietary glutamate and systemic glucose are major oxidative fuels, the PDV tissues appear to adapt to the pattern and amount of oxidative substrate supply. The preferential role for glutamate as an oxidative fuel is not completely understood. Additionally, we have previously shown (Reeds et al., 2000) that dietary glutamate is playing a quantitatively significant role in the biosynthesis of 2 conditionally essential AA (proline and arginine) and is a key factor responsible for protection of the mucosa (glutathione). Subsequent studies showed that systemic glutamine, although also a major oxidative fuel, is a poor substrate for all 3 end products. Measurements of the intestinal metabolic fate of glutamate during excessive dietary intake are therefore warranted.
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). Interestingly, although arterial lysine is taken up by the PDV, none is oxidized, suggesting a preferential oxidation of dietary lysine. As with lysine, there is significant leucine metabolism by the gut via both transamination to 
-ketoisocaproic acid and complete oxidation to CO2. Studies in piglets and dogs have demonstrated that approximately 5 to 10% of whole-body leucine flux is oxidized by the PDV (Yu et al., 1995; van der Schoor et al., 2001). Although approximately 40% of the leucine taken up by gut tissues is converted to -ketoisocaproic acid, nearly all of this is subsequently retransaminated to leucine; thus, the net release of -ketoisocaproic acid is negligible (Yu et al., 1995). Studies in young grower pigs (15 to 20 kg) suggest that approximately 40% of the whole-body phenylalanine oxidation occurs in the PDV tissues (Bush et al., 2003). This finding implies that phenylalanine hydroxylation occurs in the intestine, and is consistent with previous observations of net portal tyrosine appearance in excess of dietary intake. Given that hydroxylation rather than complete oxidation to CO2 represents the point of irreversible loss of phenylalanine, further studies are warranted to quantify the proportion of whole-body phenylalanine flux metabolized to tyrosine by the gut.
The reports of essential AA oxidation by the gut have raised the question of whether this is due to mucosal metabolism or microbial fermentation. Recent evidence of de novo lysine synthesis in the proximal intestine in pigs implicates a metabolically significant microbial flora, which could also catabolize dietary AA (Torrallardona et al., 2003a). To address this issue, emerging studies are beginning to identify the localization of essential AA catabolic enzymes within the different mucosal cell phenotypes; that is, enterocytes and lymphoid cells. Two reports have characterized branched-chain AA and lysine catabolic enzymes in enterocytes isolated from piglets at 0, 3, and 7 d of age (Elango et al., 2003; Pink et al., 2003). Branched-chain AA catabolic enzyme activity also was present in small intestinal mucosal tissue of 4-wk-old piglets (Burrin et al., 2003). These findings suggest that the lysine and leucine oxidation reported in vivo in piglets is mediated partially by mucosal metabolism; however, the relative significance of microbial catabolism remains to be determined.
Amino Acids as Biosynthetic Precursors
Besides their incorporation into the bulk protein pool and use as oxidative fuels, AA are metabolized by splanchnic tissues into a variety of end products, which serve a variety of key functions for the cell specifically, and the host in general. Studies have shown that neonatal small intestine is an important site of arginine and proline synthesis (Murphy et al., 1996; Wu, 1998; Stoll et al., 1999a; Bertolo et al., 2003). In the suckling pig, the intestinal synthesis of arginine provides only about half the animal’s needs for growth and the arginine concentration in sow’s milk is limiting (Davis et al., 1994). Thus, supplementation of dietary arginine is considered essential for maximal growth in piglets. While intestinal synthesis of arginine declines, arginase activity and synthesis rates of citrulline and ornithine from proline and arginine increase during the suckling period until after weaning. (Wu, 1997; Wu and Morris, 1998). Thus, in weaning pigs and adult rats, the intestinal conversion of glutamine, glutamate, and proline to citrulline provides a critical precursor for arginine synthesis in the kidney (Windmueller and Spaeth, 1980; Dugan et al., 1995). The dependence on intestinal first-pass metabolism for the synthesis of arginine (in neonates), or its immediate renal precursor citrulline (in adults), results in arginine deficiency when intestinal metabolism is bypassed during total parenteral nutrition (Brunton et al., 1999) or the intestine is removed surgically by resection (Wakabayashi et al., 1995).
Ornithine derived from arginine is a precursor for polyamines in intestinal cells (Blachier et al., 1995). Polyamines are ubiquitous cationic amines involved in cell proliferation and differentiation in many tissues, including the gastrointestinal tract. Polyamine synthesis from arginine is negligible in enterocytes of newborn and suckling animals (Blachier et al., 1991, 1992) because polyamines are present in mammalian milk (Pollack et al., 1992; Buts et al., 1995). The induction of intestinal polyamine synthesis from ornithine, arginine, and proline becomes physiologically significant during the weaning period, when ingestion of milk-borne polyamines ceases (Wu et al., 2000a,b).
Nitric oxide (NO) is another important end product of arginine metabolism whereby arginine is converted to citrulline. As a major physiological regulator in the body, NO enhances vascular function, tissue perfusion, and immune function. In the whole body, the proportion of arginine that is converted to NO is relatively low (1 to 10% of arginine flux; Castillo et al., 1996), but is increased under conditions of stress and trauma (Argaman et al., 2003). However, the first-pass splanchnic use of enteral arginine is about 40%, of which metabolism to NO represents 16% of the whole-body nitrate production (Castillo et al., 1993a,b). The rates of liver and PDV NO production have been determined by quantifying the conversion of 15N-arginine to 15N-citrul-line (Luiking and Deutz, 2003). These studies showed that splanchnic NO production consumes as much as 35% of utilized arginine in endotoxemic pigs; NO synthesis is also increased markedly with supplemental arginine (Bruins et al., 2002a,b).
The nonessential AA aspartate, glutamine, and glycine are key precursors for the synthesis of nucleotides. Glutamine serves as nitrogen donor for purine and pyrimidine synthesis, whereas most of carbon skeleton of nucleotides is derived from aspartate and glycine. Research results regarding the relative contribution of de novo synthesis and salvage pathways are controversial, but the relative contribution seems to be affected by the position of enterocytes within the crypt–villus axis as well as the dietary intake of nucleic acids and glutamine (see review by McCauley et al., 1998). This fact is quantitatively important in the intestinal mucosa given the high rate of cell proliferation coupled with the fact that in mice, a large portion of the nucleotides (at least ribonucleotides) are synthesized de novo (Zaharevitz et al., 1992; Boza et al., 1996). Studies in ruminants indicate that approximately 1 to 5% of the 15N-glutamine that is utilized by the gut is incorporated into RNA and DNA (Gate et al., 1999).
The biochemical mechanism whereby glutamine affects intestinal function may be related to its conversion to glucosamine, which reduces cellular NADPH and suppresses NO synthesis (Wu et al., 2001). In immune cells, glutamine and glutamate also provide an important source of NADPH via conversion of malate to pyruvate; NADPH is critical in these cells for production of superoxide and NO and for glutathione reductase activity (Newsholme et al., 2003).
Amino acids also serve as precursors for synthesis of compounds involved in support of innate immunity of the intestine and antioxidant function in both, liver and intestine. Dietary threonine and cysteine are important for mucin synthesis by goblet cells within the stomach and the intestinal mucosa. The secretory mucins play a key role in the innate immune defense of the mucosa, and the core protein of the major intestinal mucins contains a large amount of threonine and cysteine (Van Klinken et al., 1997; Faure et al., 2002). Studies in piglets indicated as much as 60% first-pass use of dietary threonine by the PDV (Stoll et al., 1998). Consistent with these findings, the threonine requirement of piglets maintained by total parenteral nutrition was <50% of that of piglets receiving enteral feedings (Bertolo et al., 1998). A subsequent report found that feeding threonine-deficient diets to piglets significantly reduces intestinal mass and goblet cell numbers; this suppression of intestinal growth cannot fully restored by providing threonine parenterally (Ball et al., 1999). The mucosal synthesis and secretion of mucins may be quantitatively significant, and therefore, the needs for dietary threonine and cysteine are likely to be increased under conditions of gut hypersensitivity or inflammation.
In addition to mucins, methionine and cysteine serve as biosynthetic precursors for numerous functional end products, including glutathione, polyamines, and taurine. In numerous cells within the body, methionine is metabolized via transmethylation to homocysteine and in the process produces S-adenosylmethionine, which donates an aminopropyl moiety in the formation of the polyamines spermidine and spermine (Figure 5; Finkelstein, 2000). Homocysteine is converted to cysteine via transsulfuration. Cysteine is one of the constituent AA of glutathione, along with glutamate and glycine. Moreover, cysteine can be metabolized to form taurine. Glutathione is a major cellular antioxidant found in cells of the intestinal mucosa and liver (Lash et al., 1986; Martensson et al., 1990). However, cysteine and taurine also can function as cellular antioxidants (Santangelo, 2002; Zafarullah et al., 2003). The quantitative significance of these functional end products to splanchnic methionine, cysteine, and glutamate use is unknown. Early studies in humans suggested that the splanchnic tissue bed is an important site of transsulfuration (Stegink and Den Besten, 1972). In piglets, first-pass use of dietary methionine ranged from 30 to 40% (Rerat et al., 1992; Stoll et al., 1998; Bos et al., 2003). A recent study in neonatal pigs showed that methionine requirement was 30% lower in parenterally fed compared with enterally fed animals, which implies that splanchnic first-pass metabolism accounts for 30% of the dietary methionine requirement (Shoveller et al., 2003).
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demonstrated that gastrointestinal tissues possess the enzymes necessary to metabolize methionine to cysteine, albeit at significantly lower activities than the liver. Recent studies based on enzyme assays and in vivo isotopic tracers in ruminants indicate that transmethylation occurs in the ruminant gut and that the activities are comparable to those in the liver (Lobley et al., 1996a, 2003; Lambert et al., 2002). Similarly, studies in piglets using dual stable isotope labeling techniques (1-13C- and 2H3-methionine) demonstrated that the PDV tissues represent a significant site of transmethylation and transsulfuration, and these pathways account for most of the methionine metabolized by these tissues (Burrin et al., 2005; Figure 5). Although transmethylation and incorporation into protein are both reversible and dynamic processes, conversion to cysteine via transsulfuration represents a net loss of methionine. The absorption of dietary cysteine into portal blood is very limited (less than 20% of dietary intake) in young pigs, implying extensive intestinal use of cysteine in first-pass (Rerat et al., 1992; Stoll et al., 1998; Bos et al., 2003). Rodent studies using 1-14C-labeled cysteine demonstrated significantly greater 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 (Stipanuk and Rotter, 1984). More recently, using stable isotope-labeled 15N-cysteine, it was demonstrated that an important metabolic fate of cysteine in intestinal tissues was its incorporation into glutathione (Malmezat et al., 2000). With respect to glutamate, studies in piglets showed that enteral glutamate is preferentially incorporated into mucosal glutathione (Reeds et al., 1997).
Amino Acids and Intestinal Microbiota
It is generally held that AA that pass from the terminal ileum into the cecum and large intestine are catabolized by microbial fermentation. The assumption that the endogenous AA are fermented and lost in the large intestine is based on early reports that colonic absorption of AA is limited and only occurs during early post-natal development (Fuller and Reeds, 1998). The carbon from colonic microbial AA catabolism can be lost as CO2 or reabsorbed into the portal blood in the form of short-chain fatty acids. However, the nitrogen released by microbial AA catabolism may be in the form of ammonia, which can be absorbed and recycled into the body AA and urea pools. Historically, the colonic microbial catabolism of essential AA has been considered a nutritional loss, because by definition, if an AA is essential, it cannot be synthesized by mammalian cells. This concept has recently been challenged by a number of elegant studies in pigs and humans, which demonstrated the microbial synthesis of some essential AA, particularly lysine, based on labeling with 15N-labeled ammonia and urea (Figure 6; Metges, 2000; Torrallardona et al., 2003a,b). These studies have shown that microbial lysine synthesis occurs in the upper gastrointestinal tract, including the small intestine, where AA are readily absorbed. Moreover, these studies suggest that microbial synthesis of several essential AA in the host gut may be nutritionally significant.
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TopAbstractINTRODUCTIONTERMINOLOGYMETABOLIC FATE OF AMINO...SUMMARY AND FUTURE PERSPECTIVESLITERATURE CITED |
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Footnotes |
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2 This work was supported by federal funds from the U.S. Department of Agriculture, Agricultural Research Service under Cooperative Agreement Number 58-6250-6-001, and by National Institutes of Health grant HD33920 (to DGB). The contents of this publication do not necessarily reflect the views or policies of the U.S. Department of Agriculture, nor does mention of trade names, commercial products, or organizations imply endorsement by the US Government.
3 Corresponding author: bstoll{at}bcm.tmc.edu
Received for publication August 30, 2005. Accepted for publication November 23, 2005.
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TopAbstractINTRODUCTIONTERMINOLOGYMETABOLIC FATE OF AMINO...SUMMARY AND FUTURE PERSPECTIVESLITERATURE CITED |
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