Modification of isoleucine-16 acetylated delta-chymotrypsin

Activation of acetylated chymotrypsinogen with trypsin leads to catalytically active acetylated delta-chymotrypsin containing NH2-terminal isoleucine. The importance of the cationic terminus to the control of the active conformation of acetylated delta-chymotrypsin has been demonstrated (Oppenheimer, H. L., Labouesse, B., and Hess, G. P. (1966) J. Biol. Chem. 241, 2720). Later studies appeared to suggest that the modification of isoleucine-16 of delta-chymotrypsin is not accompanied by the loss of catalytic activity as measured by the hydrolysis of N-acetyl-L-tyrosine ethyl ester (Agarwal, S. P., Martin, C. J., Blair, T. T., and Marini, M.A. (1971)Biochem. Biophys. Res. Commun. 43, 510; Blair, T. T., Marini, M. A., Agarwal, S. P., and Martin, C. J. (1971) FEBS Lett. 1486) or by the loss of active site content (Ghelis, C., Garel, J. R., and Labouesse, J. (1970) Biochemistry 9, 3902). In the present studies, controlled acetylation of the terminal alpha-aminogroup of acetylated delta-chymotrypsin with acetic anhydride led to a progressive loss of active sites of the enzyme. Determination of the catalytic and kinetic properties of the modified enzyme with the specific ester substrate N-acetyl-L-tyrosine ethyl ester or the nonspecific substrates p-nitrophenyl acetate and cinnamyol imidazole gave nearly identical results. With N-acetyl-L-tyrosine ethyl ester as substrate, the Km (app) values for acetylated delta-chymotrypsin (1.0 plus or minus 0.1 mM) and the modified enzyme (0.67 plus or minus 0.05 mM) are nearly identical and the kcat value is reduced to about 25% in the latter enzyme species. This value correlates well with about 20% of the active sites in this enzyme as measured by the rapid initial liberation of p-nitrophenol. With p-nitrophenyl acetate as substrate, the acylation rate constants (0.13 plus or minus 0.04 s(-1) at pH 6.0, 25 degrees, in 3.3% acetonitrile) and the deacylation rate constants (0.01 s(-1) at pH 8.5, 25 degrees, in 3.3% acetonitrile) are identical for the acetyl isoleucine-16 and the isoleucine-16 enzymes. Furthermore, the residual enzyme activity could be correlated well with the residual NH2-terminal isoleucine content and with the moles of [1--14C]acetyl groups incorporated per mol of the enzyme. The activity associated with the modified enzyme can be attributed to the enzyme species in which isoleucine-16 of acetylated delta-chymotrypsin is not acetylated. These data are in general agreement with the studies of Ghelis et al. (1970) but are in disagreement with the results of Blair et al. (1971) and of Agarwal et al. (1971) and confirm the hypothesis that the final conformation of acetylated delta-chymotrypsin containing an acetylated NH2 terminus is catalytically inactive and resembles acetylated zymogen in many of its physical properties.