The dual effects of alcohols on the kinetic properties of guinea pig liver cytosolic β-glucosidase

This report demonstrates the effect of primary alcohols on the kinetic properties of guinea pig liver cytosolic beta-glucosidase. Lineweaver-Burk analyses of the kinetic data revealed a biphasic response; at low concentrations the alcohols increased the Vmax 5--7-fold while at higher concentrations they caused a purely competitive type of inhibition. For example, with n-butyl alcohol, increasing the alcohol's concentration in the assay medium from 0 to 0.14 M (0-1% (v/v)) resulted in a progressive increase in Vmax to a value 7-fold above the basal level without affecting the Km. However, between 0.14 and 0.54 M (1 and 4% (v/v)) n-butyl alcohol, the Km for 4-methylumbelliferyl-beta-D-glucopyranoside increased significantly from 0.14 to 0.93 mM. In contrast to n-butyl alcohol or isobutyl alcohol, which are potent activators, structurally related compounds like sec-butyl alcohol, tert-butyl alcohol, butylurea, and butanesulfonic acid did not stimulate the activity of the cytosolic beta-glucosidase. In the concentration range where activation was observed, conventional secondary replots of 1/delta slope versus 1/[alcohol] yielded perfect straight lines, demonstrating that binding of a single molecule of alcohol to the beta-glucosidase was responsible for the initial phase of activation. Furthermore, the glycohydrolase displayed a propensity to bind the longer chain alcohols, as reflected by the KA (binding constant) values of 555, 146, 34.1, and 7.47 mM for ethanol, n-propyl alcohol, n-butyl alcohol, 1-pentanol, respectively. This phenomenon of nonessential activation by alcohols has led us to speculate on the presence of a physiologic activator for the beta-glucosidase in mammalian tissues which contain this enzyme.