Comparison of some characteristics of membrane-bound neutral proteinase activity in the microsomal fractions of rat kidney and small intestine

Some characteristics of the membrane-bound neutral proteinase activity in the microsomal fractions of rat kidney and small intestine were compared, using heatdenatured casein as a substrate. The proteinases of both kidney and small intestine showed maximal activity between pH 8.0 and 8.5, and were strongly inhibited by EDTA, o-phenanthroline, p-chloromercuriphenyl sulfonate, dithiothreitol, and chymostatin. Phosphoramidon and other reagents tested, including deoxycholate and bestatin, which strongly inhibited the contaminating aminopeptidase activity, were without marked effect on the neutral proteinase activity. Among ureadenatured proteins tested as substrates, casein, histone, and hemoglobin were hydrolyzed rapidly by both proteinase preparations. Fibrinogen was a good substrate for the kidney enzyme whereas it was not hydrolyzed well by the small intestine proteinase. On the other hand, serum albumin was hydrolyzed well by the small intestine proteinase, but not by the kidney proteinase. These results indicate that the neutral proteinase activity of the microsomal membrane fractions is largely due to metalloproteinases, which are quite similar, but not identical, in the kidney and small intestine.