Evidence for an Enzyme which Cleaves the Guanidinobenzoate Moiety from Active‐Site Titrants Specifically Designed to Inhibit and Quantify Trypsin

Abstract

The serum and ascitic plasma surrounding Ehrlich ascites tumour cells grown in mice contains an enzyme which cleaves 4‐nitropheny‐ p ‐guanidinobenzoate and 4‐methylumbelliferyl‐ p ‐guanidinobenzoate. We refer to this enzyme as guanidinobenzoatase. Either of the substrates acts as a competitive substrate for the enzyme when it is assayed with the other substrate. Arginyl‐containing peptide substrates of typsin and thrombin also act as competitive inhibitors of the enzyme but are not cleaved by it. The properties of this enzyme are remarkable in the continuous production of cleavage products from the two substrates, which are both active‐site titrants for trypsin‐like enzymes, rather than stoichiometric cleavage and inhibition of one molecule of enzyme by one molecule of active‐site titrant. The enzyme contains serine in its active centre, being inhibited by both phenylmethylsulphonyl fluoride and diisopropyl‐fluorophosphate.