Peptide hydrolase activities of the mucosa of human small intestine

310

163

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.

Section: Discussionmentioning

confidence: 53%

Section: Introductionmentioning

confidence: 98%

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

Adibi¹,

Morse²

1971

310

163

“…3 Other substrates used were L-leucyl-p-naphthylamide hydrochloride2 and L-leucinamide hydrochloride. 2 Trypsin,' chymotrypsin,' papain,5 Triton X-100,6 Na deoxycholate,' Na glycodeoxycholate,' Na glycocholate,' and Enzite-papain 8 were used to release peptide hydrolases from the brush border membrane. Hydrolyzed starch was used for the preparation of the gel used in electrophoresis.…”

Section: Methodsmentioning

confidence: 99%

“…These enzymes have been reported to occur both in brush border membranes and in the soluble cytoplasmic fraction (1)(2)(3)(4)(5)(6). Although data regarding the quantitative distribution of peptide hydrolases in these fractions are lacking, it has been reported that the brush border membrane contains less dipeptide hydrolase activity than the soluble cytosol fraction, while tripeptide hydrolase activity is much more active in the former (4,5).…”

Section: Introductionmentioning

confidence: 99%

Peptide hydrolases in the brush border and soluble fractions of small intestinal mucosa of rat and man

Kim¹,

Birtwhistle²,

Kim³

1972

160

A BST RA CT Peptide hydrolases, catalyzing the hydrolysis of 13 dipeptides and 5 tripeptides into their respective amino acids, were studied in small intestinal mucosa and other tissues, in man and in the rat.Studies on the subcellular distribution of these enzymes showed enzyme activities in both the soluble and brush border fractions of the rat small intestinal mucosa, the former constituting 80-90% and the latter 10-15% of the total activity. Zymogram studies of peptide hydrolases, in both fractions, yielded multiple bands indicating multiple zones of enzyme activity. With most substrates a rather broad range of enzyme activities was observed in the soluble fraction differing only slightly from substrate to substrate, the exception being when L-leucyl-L-proline was used: this latter led to a zymogram pattern which was quite distinct. The synthetic substrates, L-leucyl-P-naphthylamide and Lleucinamide appeared to be hydrolyzed by two electrophoretically distinct enzymes, different from those hydrolyzing other leucyl-containing peptide substrates.Zymogram patterns of the brush border membrane fraction were quite different from those of the soluble fraction of rat small intestine indicating that enzymes from the two sources may be different. No comparable human data were obtained.Peptide hydrolases in the soluble fractions of various organs from the same species gave similar zymogram patterns, while those from the plasma membrane-bound fractions of different organs in the same species were peculiar to each organ. From these data, it is suggested that peptide hydrolases in the brush border and the soluble fractions of small intestine are distinct enzymes and may play different roles in cellular function.

“…Since the Matheson and Tattrie method gave high blank values when these dipeptides were used as substrates, a one-step modification' of previously reported L-amino acid oxidase methods was used for assays of peptide hydrolase activities (16,22,23). Each dipeptide was dissolved in 0.5 ml 50 mM Tris-HCl buffer, pH 8.0.…”

Section: Experiments Imentioning

confidence: 99%

The Responses of Rat Intestinal Brush Border and Cytosol Peptide Hydrolase Activities to Variation in Dietary Protein Content DIETARY REGULATION OF INTESTINAL PEPTIDE HYDROLASES

Nicholson¹,

McCarthy²,

Kim³

1974

AB STR A CT The effects of variation in dietary protein content on small intestinal brush border and cytosol peptide hydrolase activities have been investigated.One group of rats was fed a high protein diet (55% casein) and another group was fed a low protein diet (10% casein). After 1 wk, brush border peptide hydrolase activity (L-leucyl-P-naphthylamide as substrate) and cytosol peptide hydrolase activity (L-prolyl-L-leucine as substrate) were determined in mucosae taken from the proximal, middle, and distal small intestine. As judged by several parameters, brush border peptide hydrolase activity was significantly greater in rats fed the high protein diet when data for corresponding segments were compared. In contrast, no significant difference was seen in cytosol peptide hydrolase activity.In a second study, brush border and cytosol peptide hydrolase activities were determined in the proximal intestine by utilizing an additional three peptide substrates: L-leucyl-L-alanine, L-phenylalanylglycine, and glyCyl-L-phenylalanine. Sucrase, maltase, and alkaline phosphatase activities were also determined. As before, brush border peptide hydrolase activities were significantly greater in rats fed the high protein diet. However, activities of the nonproteolytic brush border enzymes did not vary significantly with diet. In contrast to the results obtained with L-prolyl-L-leucine as substrate for the cytosol enzymes, cytosol activity against