Stimulation of hepatic microsomal beta-glucuronidase by calcium.

Hydrolysis of 3-methylumbelliferyl glucuronide by liver microsomal beta-glucuronidase is enhanced about 2-fold by micromolar concentrations of Ca2+; half-maximal stimulation occurs with 0.35 microM Ca2+. Dissociation of the enzyme from microsomal membranes by various treatments increases basal beta-glucuronidase activity and markedly decreases the sensitivity of the enzyme to Ca2+. Under similar conditions, the soluble lysosomal form of the enzyme is insensitive to Ca2+. Ca2+ stimulation was unaltered by addition of calmodulin inhibitors or exogenous calmodulin. Thus, interaction of cytosolic Ca2+ with membrane bound beta-glucuronidase may modulate glucuronidation in intact hepatocytes via a novel, calmodulin-independent mechanism.