Two broad-specificity dipeptide hydrolysing activities from cytoplasm of guinea pig brain, both of which contain prolinase but neither of which contain carnosinase activity

Two dipeptide hydrolysing activities were purified from cytoplasm of guinea pig brain. Dipeptide hydrolase I has been shown to be a strict dipeptidase requiring a free amino and a free carboxy terminus, while dipeptide hydrolase II displays very low activity against Leu–Leu–Leu. Of the 41 dipeptides presented to both enzymes, 25 were hydrolysed by both enzymes, while six (including carnosine) were hydrolysed by neither. Six were hydrolysed solely by dipeptide hydrolase I and four were hydrolysed solely by Pro–Leu hydrolase II. Kinetic analysis suggested that dipeptides which were hydrolysed with unfavourable kinetics or which were not hydrolysed by one dipeptide hydrolase were generally hydrolysed with more favourable kinetics by the other dipeptide hydrolase. Dipeptide hydrolase I displays optimum activity at pH 9.0, while dipeptide hydrolase II was optimally active at pH 8.0. Both enzymes were inhibited by 1,10-phenanthroline, p-chloromercuribenzoate and bestatin. Dipeptide hydrolase II was more strongly inhibited by arphamenine B than was dipeptide hydrolase I. Dipeptide hydrolase II was also inhibited by N-ethyl maleimide, while dipeptide hydrolase I was inhibited by dithiothreitol. Native M r values of 70,000 and 67,000 were computed for dipeptide hydrolase I and dipeptide hydrolase II, respectively. Sodium dodecyl sulphate polyacrylamide gel electrophoresis conducted with each enzyme under denaturing conditions suggested that both enzymes were comprised of a single polypeptide chain.