Acid proteases. II. Fluorescence study of the interaction of Cladosporium acid protease with glycyl-DL-norleucine methyl ester in the presence of cupric ions.

Glycyl-DL-norleucine methyl ester (GN), a diazoacetyl-DL-norleucine methyl ester (DAN) analog, in the presence of cupric ions was found to partially quench the protein fluorescence of acid protease from Cladosporium sp. No. 45-2, and cupric ions were also found to quench the fluorescence. These quenchings were pH-dependent. GN alone did not quench the fluorescence of the enzyme. The interaction between the enzyme and GN in the presence of cupric ions was studied statically at pH 5.4 in terms of fluorescence change. The dissociation constant, Kd, of the enzyme-GN complex in the presence of a 20-fold molar excess of cupric ions (0.08 mM) determined by fluorescence titration at 30 degrees C (Kd = 1.86 mM) was in good agreement with that obtained for GN from kinetics of inhibition of DAN-induced inactivation in the presence of a 20-fold molar excess of cupric ions at 30 degrees C (KA = 1.94 mM) (Kanazawa, H. (1977) J. Biochem. 81, 1739-1744). At various concentrations of cupric ions, no change of Kd was found. These results suggest that cupric ions are attracted to a negatively charged carboxyl group responsible for the formation of the enzyme-GN complex.