Effects of ethanol on cytoplasmic peptidases of the jejunal epithelial cell of the hamster

Abstract

Although ethanol has been reported to inhibit intestinal amino acid absorption and peptide hydrolysis by the brush border membrane (BBM) peptidases, its effect on other events of protein absorption (such as peptide hydrolysis by cytosol peptidases, absorption of peptides across the BBM, and translocation of amino acids across the basolaterial membrane) has not yet been reported. To obtain a better understanding of the overall effect of ethanol on intestinal protein absorption, in the present study we have investigated the influence of ethanol on the cytosol peptidases. In order to examine the activity of these enzymes, without the influence of brush border digestion and translocation of peptides, the present study was carried out in vitro using a preparation of cytosol peptidases. Results show that exposure of the enzymes to 1-5% (w/v) ethanol caused a dose-dependent inhibition of hydrolysis of L-leucylglycine (Leu-Gly), glycyl-L-tyrosine (Gly-Tyr), and L-phenylalanylglycine (Phe-Gly) by the cytosol peptidases. These inhibitions were completely reversible. Kinetic studies indicated that ethanol depressed the hydrolysis of Leu-Gly and Gly-Tyr by a mixed type of inhibition, in which the Vmax decreased and the Km increased. In the hydrolysis of Phe-Gly, two enzymes were involved, and ethanol depressed the Vmax of both, without affecting their Km. These findings suggest that ethanol alters only the catalytic center of both enzymes involved in the hydrolysis of Phe-Gly and alters both the catalytic center and the substrate binding site of the enzymes involved in the hydrolysis of Leu-Gly and Gly-Tyr. The results of this study together with those of our previous investigation on BBM peptidases indicate that ethanol interferes with the intestinal hydrolysis of peptides and, therefore, probably with the absorption of protein.