Kinetics of hydrolysis of phenylthiazolones of arginine, homoarginine, norarginine, and canaavanine by trypsin

Phenylthiazolones (PTAs) of arginine and its homologs and analogs, homoarginine, norarginine (alpha-amino-gamma-guanidinobutyric acid), canavanine, and gamma-hydroxyarginine, were prepared. A steady-state kinetic analysis of the trypsin [EC 3.4.21.4]-catalyzed hydrolysis reactions was carried out and the kinetic parameters for these internal thioesters were compared with those for normal linear ester substrates. PTA-gamma-hydroxyarginine was so labile that hydrolysis by the enzyme could not be followed. PTA-arginine has a specificity constant (Kcat/Km) comparable to that for the Nalpha-unblocked arginine ester substrate, though the value is about 0.1% of that for a specific ester substrate, Nalpha-tosylarginine methyl ester. PTA derivatives of canavanine and homoarginine were hydrolyzed with Kcat/Km walues of the same order of magnitude as that for PTA-arginine. However, PTA-noraginine was much less susceptible to tryptic hydrolysis that PTA-homoarginine, while the linear esters of norarginine are known to be more susceptible than those of homoarginine.