Substrate specificity and inhibitor sensitivity of a trypanosomatid alkaline peptidase

An alkaline peptidase of Trypanosoma cruzi and Crithidia fasciculata, previously shown to cleave on the carboxyl side of arginine and lysine residues, was examined for its ability to cleave various fluorogenic substrates and for its sensitive to peptidase inhibitors. The enzyme of both T. cruzi and C. fasciculata has a preference for cleavage of substrates with basic amino acids at the P2 as well as the P1 position of the peptide chain. Arginine and lysine are equally acceptable at P2, whereas the enzyme prefers arginine to lysine at P1. An influence of the P3 amino acid residue on substrate cleavability was also apparent. The peptidase was highly susceptible to inactivation by peptidylfluoromethanes, peptidyldiazomethanes and peptidylsulphonium salts that contained arginine or lysine at P1. Additionally, diisopropylfluorophosphate inhibited the enzyme, whereas trans-epoxysuccinylleucylamido(4-guanidino)butane and iodoacetic acid were relatively weak inhibitors. Various reversible inhibitors of the enzyme were also examined. Inhibition by members of the primary aliphatic amine series, methylamine to n-heptylamine, showed a peak of inhibition at n-butylamine, which most closely resembles the lysine side chain. Agmatine, which resembles the arginine side chain, also strongly inhibited the peptidase. The kinetics of inhibition by these basic compounds were of the competitive type. Pentamidine and hirudonin, which resemble two arginine side chains joined together, were more effective inhibitors of the trypanosomatid peptidase than bases resembling only one arginine or lysine side chain.