Active-Site-Directed Inactivation of Aspergillus oryzae β-Galactosidase with β-D-Galactopyranosylmethyl-pnitrophenyltriazene

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Abstract
β-D-Galactopyranosylmethyl-p-nitrophenyltriazene (β-GalMNT), a specific inhibitor of β-galactosidase, was isolated as crystals by HPLC and its chemical and physicochemical characteristics were examined. Aspergillus oryzae β-galactosidase was inactivated by the compound. We studied the inhibition mechanism in detail. The inhibitor was hydrolyzed by the enzyme to p-nitroaniline and an active intermediate (β-galactopyranosylmethyl carbonium or β-galactopyranosylmethyldiazonium), which inactivated the enzyme. The efficiency of inactivation of the enzyme (the ratio of moles of inactivated enzyme to moles of β-GalMNT hydrolyzed by the enzyme) was 3% the efficiency of Escherichia coli β-galactosidase was 49%. In spite of the low efficiency, the rate of inactivation of A. oryzae enzyme was not very different from that of the E. coli enzyme, because the former hydrolyzed β-GalMNT faster than the latter did. A. oryzae β-galactosidase was also inactivated by p-chlorophenyl, p-tolyl, and m-nitrophenyl derivatives of β-galacto-pyranosylmethyltriazene. However, E. coli β-galactosidase was not inactivated by these triazene derivatives. The results showed that the inactivation of A. oryzae and E. coli β-galactosidases by β-GalMNT was an enzyme-activated and active-site-directed irreversible inactivation. The possibility of inactivation by intermediates produced nonen-zymatically was ruled out for E. coli, but not for the A. oryzae enzyme.