AJP - Gastrointestinal and Liver Physiology, Vol 255, Issue 2 212-G220, Copyright © 1988 by American Physiological Society
Protein digestion in human and rat small intestine: role of new neutral endopeptidases
D. Guan, M. Yoshioka, R. H. Erickson, W. Heizer and Y. S. Kim
Gastrointestinal Research Laboratory, Veterans Administration Medical Center, San Francisco 94121.
Two new phosphoramidon-insensitive neutral endopeptidases were identified
and partially characterized in the brush-border membrane of rat and human
intestine using N-CBZ-L-Ala-L-Arg-L-Arg-4-methoxy-beta-naphthylamide
(Z-Ala-Arg-Arg-MNA) and azocasein or alpha-casein as substrates. Activities
in the brush-border membrane of both rat and human intestine were maximum
at neutral to alkaline pH, were inhibited by metal chelating and thiol
reagents, and were insensitive to phosphoramidon. The results also indicate
that these endopeptidases are distinct from pancreatic proteases. The
biochemical properties of the enzyme hydrolyzing Z-Ala-Arg-Arg-MNA were
shown to be different from that hydrolyzing azocasein or alpha-casein.
Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel
filtration revealed that several native intact protein substrates were
rapidly degraded to small molecular weight peptides and amino acids when
incubated with rat or human brush-border membrane preparations. During in
vivo intestinal perfusion in rats, 11% of the total administered
alpha-casein was hydrolyzed and absorbed by the intestine. The results
suggest that phosphoramidon-insensitive endopeptidases in the intestinal
brush-border membrane may be of nutritional and physiological importance in
protein digestion.
