Human lysosomal β-glucosidase: Kinetic characterization of the catalytic, aglycon, and hydrophobic binding sites

Three binding sites on highly purified lysosomal β-glucosidase from human placenta were identified by studies of the effects of interactions of various enzyme modifiers. The negatively charged lipids, taurocholate and phosphatidylserine, were shown to be noncompetitive, nonessential activators of 4-methylumbelliferyl-β-d-glucoside hydrolysis. Similar results were observed using the natural substrate, glucosyl ceramide, and low concentrations of taurocholate (<1.8 mm) or phosphatidylserine (0.5 mm). However, higher concentrations resulted in a complex partial inhibition of glucosyl ceramide hydrolysis. Increasing concentrations of phosphatidylserine obviated the effects of taurocholate, suggesting that these compounds compete for a common binding site on the enzyme. Glucosyl sphingosine and its N-hexyl derivative were potent noncompetitive inhibitors of the enzyme activity using either substrate. Taurocholate (or phosphatidylserine) and glucosyl sphingosine were shown to be mutually exclusive, indicating competition for a common binding site. In contrast, octyl- and dodecyl-β-glucosides were linear-mixed-type inhibitors of glucosyl ceramide or 4-methylumbelliferyl-β-d-glucoside hydrolysis, indicating at least two binding sites on the enzyme. Inhibition by these alkyl β-glucosides was observed only in the presence of taurocholate or phosphatidylserine. The competitive component [K_i (slope)] for the two alkyl β-glucosides decreased with increasing alkyl chain length, and was unaffected by increasing taurocholate or phosphatidylserine concentration. The noncompetitive component [K_i (intercept)] was nearly identical for both alkyl β-glucosides and was decreased by increasing taurocholate or phosphatidylserine concentration. These results indicated that the negatively charged lipids and alkyl β-glucosides were not mutually exclusive, but interacted with different binding sites on the enzyme. Gluconolactone was shown to protect the enzyme from inhibition by the catalytic site-directed covalent inhibitor, conduritol B epoxide, indicating an interaction at a common binding site. In the presence of substrate, taurocholate facilitated the inhibition of gluconolactone or conduritol B epoxide. These studies indicated that lysosomal β-glucosidase had at least three binding sites: (i) a catalytic site which cleaves the β-glucosidic moiety, (ii) an aglycon site which binds the acyl or alkyl moieties of substrates and some inhibitors, and (iii) a hydrophobic site which interacts with negatively charged lipids and facilitates enzyme catalysis.