Brush border and cytosol peptidase activities of human small intestine in normal subjects and celiac patients

Abstract

Peptidase activities have been investigated in the brush border of human proximal jejunum by using dipeptides and tripeptides and β-naphthylamides of glycyl-L-proline and amino acids as substrates. The activities hydrolyzing glycyl-L-leucine, L-phenylalanyl-L-alanine, and L-leucvlglycylglycine in the brush border were found to be only 1.5. 15, and 16% of the total peptidase activities present in the intestinal mucosa, but the specific activities for the hydrolysis of these substrates appeared in the brush border to be as high as or higher than that of sucrase. The enzyme(s) hydrolyzing L- phenylalanyl-L-alanine in the brush border showed different properties from the enzyme(s) hydrolyzing the same substrate in the cytosol, the former being completely resistant to ρ-hydroxymer- curibenzoate, partially resistant to heating, and inhibited by puromycin by about 50%. On the other hand, the enzymatic activities hydrolyzing the β-naphthylamides of glycyl-L-proline, L-leucine, and α-L-glutamic acid as well as N-carbobenzoxy-L-prolyl-L-alanine were shown to be almost totally localized in the brush border. All the peptidase and β-naphlliylamidasc activities studied were well solubilized by papain from the brush border membrane with the only exception being the activity hydrolyzing glycyl-L-leucine. By acrvlamide gel electrophoresis, three enzymatic activities were clearly separated from each other as well as from the oligoami- nopeptidase- (EC 3.4.11.2) splitting L-leucyl-β-naphthylamide: (1) the aminopeptidase A (EC 3.4.11.7) hydrolyzing α-L-glutamvl-β- naphthylamide; (2) the dipeptidylaminopcptidase IV (EC 3.4.14.-) liberating glycyl-L-proline from glycyl-L-prolyl-β-naphthylamide; and (3) a carboxypeptidase hydrolyzing N-carbobenzoxy-L-prolyl-L-alanine (EC 3.4.12.-). Brush border peptidases (oligoaminopeptidase, aminopeptidase A, dipeptidylaminopcptidase IV, and carboxypeptidase) and cytosol dipeptidase and tripeptidase activities were measured in intestinal biopsies of celiac patients utilizing specific substrates. These enzymatic activities were normal in eight children with celiac disease in histologic remission, with only aminopeptidase A being reduced to 70% of control values. On the contrary, in the atrophic mucosa of 12 children with active celiac disease, these were all significantly but not equally reduced. Speculation: The identification of the above-mentioned peptidases in the intestinal brush border demonstrates the importance of this sub- cellular organelle in the digestion of protein and peptides complementary to intraluminal and intracellular digestion. Brush border peptidases are probably involved in the digestion of gliadin, which is very rich in glutamic acid and proline residues; as these activities are lowered in the atrophic celiac mucosa, digestibility of gliadin peptides might be reduced during active celiac disease. © 1980 International Pediatric Research Foundation, Inc.