Absorption of l-methionine from the human small intestine

A B S T R A C T The intestinal fate of two tripeptides (triglycine and trileucine). which differ markedly in solubility and molecular weight, have been investigated by jejunal perfusion in healthy human volunteers. Rates of glycine or leucine uptake from test solutions containing triglycine or trileucine were greater than from test solutions containing corresponding amounts of free glycine or free leucine, respectively. The rate of glycine uptake from a 100 mM triglycine solution was greater than that from a 150 mM diglycine solution. At each infused load of triglycine (e.g., 1,000 imol/min) the rates (micromoles/minute per 30 cm) of either triglycine disappearance (810±40) or glycine absorption (2,208±+122) were markedly greater than the luminal accumulation rates of either diglycine (56±10) or free glycine (110±18). The luminal accumulation rate of free leucine during infusion of a 5 mM trileucine solution was over threefold greater than that of free glycine during the infusion of a 5 mM triglycine solution. Luminal fluid exhibited no hydrolytic activity against triglycine, but contained some activity against trileucine. Saturation of free amino acid carrier system with a large load of leucine did not affect glycine absorption rate from a triglycine test solution, but isoleucine markedly inhibited the uptake of leucine from a trileucine solution. When the carrier system for dipeptides was saturated with a large amount of glycyl-A preliminary account of this work has been previously published (1974. Gastroenterology. 66: 657) leucine, the disappearance rate of triglycine was considerably reduced while that of trileucine remained unaffected. After addition of glycylleucine to tripeptide solutions, there was a minimal increase in the luminal accumulation of diglycine, while dileucine accumulation was increased by 62-fold.These studies suggest that the modes of intestinal disappearance of the above two tripeptides are different. Triglycine is taken up intact by human jejunum; this uptake is mediated totally or partially by the carrier system which also transports dipeptides. In contrast, trileucine is hydrolyzed to leucine and dileucine mostly on the cell surface and a small fraction is hydrolyzed in the gut lumen; these hydrolytic products are then taken up by the free amino acid and dipeptide carrier systems, respectively.

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Evidence for two different modes of tripeptide disappearance in human intestine. Uptake by peptide carrier systems and hydrolysis by peptide hydrolases.

Adibi¹,

Morse²,

Masilamani³

et al. 1975

“…In recent years, the in vitro and in vivo transport of free amino acids in the intestine of experimental animals (1) as well as of man (2)(3)(4)(5)(6)(7)(8) has received considerable attention. In comparison, the intestinal fate of intermediate products of protein digestion (peptides) has not been extensively studied.…”

Section: Introductionmentioning

confidence: 99%

Intestinal transport of dipeptides in man: relative importance of hydrolysis and intact absorption

Adibi¹,

Morse²

1971

307

162

A 30 cm segment of the duodenum, jejunum, or ileum of normal human volunteers was perfused, on separate occasions, with test solutions containing either glycylglycine, free glycine, glycylleucine, or equimolar amounts of free glycine and free leucine. Luminal fluid contained no hydrolytic activity against glycylglycine and minimal activity against glycylleucine. In each intestinal segment, amino acid absorption rates were significantly greater from the test solutions containing the same amount of amino acids in dipeptide than in free form (as high as 185% increase). Perfusion of each intestinal segment with a test solution containing the equimolar mixture of free glycine and free leucine always resulted in a greater leucine than glycine absorption rate. This preferential absorption of leucine, however, was either diminished (jejunum) or almost abolished (duodenum and ileum) when the glycylleucine solution instead of the equimolar mixture was presented to the intestinal mucosa. Among the three segments, the duodenum exhibited the least potential for the disappearance of dipeptides. The jejunal and ileal dipeptide disappearance rates were either similar for glycylleucine (94% vs. 92%) or slightly different for glycylglycine (92% vs. 79%). Despite lack of a remarkable difference in the disappearance rates, absorption rates of constituent amino acids were markedly greater in the jejunum than in the ileum. This reduced amino acid absorption was brought about by a greater accumulation of free amino acids in the lumen of the ileal segment in the lumen than when the glycylglycine test solution did not contain free leucine. Similarly, inhibition of free glycine and free leucine absorption by isoleucine was not accompanied by any remarkable alteration of absorption rates of the constituent amino acids of glycylleucine. The results of these studies suggest that: (a) dipeptide disappearance in the gut lumen is principally accomplished by intact absorption and not by hydrolysis; (b) intracellular hydrolysis of dipeptides is markedly greater in the ileum than in the jejunum, while dipeptide absorption rates are either similar or only slightly different in these two segments; (c) there is no appreciable hydrolysis of glycylglycine by the membrane-bound enzymes and only a small fraction of glycylleucine is hydrolyzed by these enzymes.

“…Others have shown that unconjugated estrogens (18,19), fat (20), and manganese (21) can be found in the intestinal lumen in the absence of bile. Furthermore, recent studies on intestinal absorption in man have revealed that there are functional differences at different levels of the human small intestine (22,23): It is most likely that Es-16G1 is partly absorbed intact, presumably at the proximal segment, while the unabsorbed Es-16G1 reaches the distal segment where it is subjected to further metabolism. Thus, it appears that intestinal secretion should be considered as an additional parameter of estrogen metabolism.…”

Section: -7)mentioning

confidence: 99%

Studies on phenolic steroids in human subjects

Inoue¹,

Sandberg²,

Graham³

et al. 1969

A B S T R A C T 6,7-8H-Estriol-16a-glucosiduronate-"C was administered to eight women (nine studies) by several routes: both injection and infusion (300 min) into the cubital vein, injection into the portal vein system, ingestion and instillation into the duodenum, jejunum, and ileum. Urine, collected from 0-2, 2-4, 4-8, 8-12, and 12-24 hr, was analyzed by countercurrent distribution for its content of radioactive 3-and 16-glucosiduronate (E-3GLE-16G1) and sulfoglucosiduronate (Es-3S,16Gl) of estriol as well as for 'H/1C ratio of each conjugate. After peripheral injection 50-60% of the injected E-16G1 was excreted unchanged along with about 5% as E&-3S,16G1 with an unchanged 8H/"C ratio, indicating direct sulfation of the injected E-16G1. During a 300 min infusion, urinary excretion closely resembled that following injection. But 2-4 hr after the end of the infusion excretion of EX-3S, 16G1 stopped, excretion of E-3G1 (17%/24 hr) with an elevated 'H/'4C ratio started, and excretion of E&-16G1 continued (70%/24 hr), but with a rapidly increasing 'H/14C ratio. This indicated sequestration in a sluggishly metabolizing compartment where two processes occurred: (a), extensive hydrolysis of E.-16G1 followed by reconjugation at either C3 or C16 with unlabeled uridine diphosphate glucuronic acid (UDPGA), thereby increasing the 'H/"C ratio; and (b) transconjugation from C16 to C3, thereby producing E-3G1 with finite 8H/"C ratios. Instillation into various segments of the small intestine produced results qualitatively similar to those after intravenous infusion, whereas ingestion and intraportal injection resembled peripheral intravenous injection. Therefore, we have postulated the This work was presented at the 50th Meeting of The Endocrine Society, June 1967, Bal Harbour, Fla.Dr. Inoue is a postdoctoral fellow from the