Dipeptidylaminopeptidase and carboxypeptidase activities of the brush border of rabbit small intestine

Abstract

Two peptidases have been found in the brush border of rabbit small intestine: dipeptidylaminopeptidase IV, which is able to hydrolyze glycyl-l-proline from glycyl-l-prolyl-β-naphthylamide, and a carboxypeptidase, which is able to hydrolyze N-carbobenzoxy-l-prolyl-l-alanine and is activated by Co2+. These enzymes have the highest levels of specific activity in the distal ileum. The enzymatic activities hydrolyzing these substrates were found to be almost totally localized in the brush border: 90 and 99% of carboxypeptidase and dipeptidylaminopeptidase activities, respectively, were recovered in the brush border, assuming 100% recovery of the sucrase activity in this subcellular fraction. Both enzymes were present in the brush border of rabbit fetuses at the end of gestation as well as in the brush border of rabbits 7 days after ligation of the pancreatic duct. These results demonstrate that they are not bacterial or pancreatic enzymes absorbed on the luminal surface of the brush border. The proteins of brush border of rabbit intestine were solubilized by papain digestion and filtered on Sephadex G-200: the electrophoresis on gradient of polyacrylamide gel of the filtered proteins identified six peptidases: the dipeptidylaminopeptidase IV; the carboxypeptidase; the oligoaminopeptidase (substrate l-leucyl-β-naphthylamide); the aminopeptidase A (substrate α-l-glutamyl-β-naphthylamide); and two other peptidases hydrolyzing glycyl-l-leucine and l-methionyl-l-leucine. The demonstration in the brush border of these different peptidases suggests that this subcellular organelle plays a major role in terminal protein digestion, complementary to intraluminal digestion and intracellular digestion.