Inhibition of human liver beta-galactosidases and beta-glucosidase by n-bromoacetyl-beta-D-galactosylamine

Abstract

N-Bromoacetyl-beta-D-galactosylamine is an irreversible inhibitor of the 'acid' and the 'neutral' beta-galactosidases (beta-D-galactoside galactohydrolase, EC 3.2.1.23) of human liver. The inactivation of acid beta-galactosidase appears to involve a group with a pKa = 4.5. The inhibition of neutral beta-galactosidase only occurs above pH 8.0. Both enzymes are protected against inhibition by the presence of substrates, suggesting that the inhibitor reacts with the active site of the enzymes. Other lysosomal hydrolases are not inhibited by N-bromoacetyl-beta-D-galactosylamine, with the exception of 'neutral' beta-glucosidase (beta-D-glucoside glucohydrolase, EC 3.2.1.21). The pH dependence of neutral beta-glucosidase inactivation is essentially identical to that of the neutral beta-galactosidase. Inhibition of beta-glucosidase by this galactose derivative suggests that the same enzyme may bind glucosides and galactosides. Furthermore, both neutral beta-galactosidase and beta-glucosidase are inactivated at 52 degrees C with a half-life of 7.5 min. The presence of a single enzyme with both beta-glucosidase and beta-galactosidase activities is also supported by mixed-substrate experiments.