Inhibition of “Serine” Esterases by Phenylarsonic Acids

Chymotrypsin, trypsin, and Novo and Carlsberg subtilisins undergo a reversible, time-dependent inhibition by phenylarsonates. The inhibition of these enzymes by p-nitro-, p-tolyl-, p-aminophenyl-, and phenylarsonate was studied in detail, as a function of pH and inhibitor concentration, by a variety of approaches. Ki values were determined for the inhibition of chymotrypsin and the subtilisins by these compounds. Active site titrations and dye displacement experiments were used to show that the inhibition results from the interaction of the arsenicals with the active sites of these enzymes. The pH dependence of the inhibition of both chymotrypsin and the subtilisins followed the theoretical curve for the protonation of a group on the enzymes with a pK' value of 7.15. The results obtained in this study, taken in conjunction with earlier scattered reports in the literature, suggest that the phenylarsonates are general inhibitors of serine esterases. Possible mechanisms for such inhibition are discussed.