Lysosomal ß-Glucosidase of Rat Liver

Studies were undertaken to characterize the ß-glucosidase activity in freshly homogenized liver from Sprague-Dawley rats. About 95% of the total ß-glucosidase activity was associated with the particulate fraction, whereas only about 3-7% was found in the cytosol. Storage of fresh liver at room temperature for several hours or repeated freezing and thawing of fresh rat liver prior to homogenization, solubilized 20-30% of the total hepatic ß-glucosidase activity. An additional 30% could be solubilized by extracting the particulate sediments with water or Triton X-100. The enzymatic activity in both the particulate and solubilized fractions optimally hydrolyzed 4-methylumbelliferyl-ß-D-glucoside as well as the glycolipid substrate, glucosylceramide, at an acidic pH. The rates of hydrolysis of either substrate by all subcellular fractions were stimulated by addition of sodium taurocholate or phosphatidylserine. The particulate, cytosolic and solubilized enzymes bound to concanavalin A, were inhibited by conduritol B epoxide and migrated more electronegatively on cellulose acetate than the cytosolic acid ß-glucosidase from human liver or spleen. These data indicated that the liver of Sprague-Dawley rats contained primarily the lysosomal acid ß-glucosidase (‘glucocerebrosidase’) and little, if any, ‘nonspecific’ ß-glucosidase. This, and the fact that about 60% of the rat hepatic ß-glucosidase could be solubilized by autolysis, freezing and rethawing or extraction with water, contrasts with the ß-glucosidases in human liver since about 80% of the total ß-glucosidase activity is cytosolic and does not hydrolyze glucosylceramide. Thus, rat liver ß-glucosidase provides a useful source for studies of the lysosomal ß-glucosidase, uncontaminated with other enzymes of similar activity. Also, the fact that a major portion of the enzyme could be solubilized without the use of detergents suggests that in rat liver the lysosomal ß-glucosidase is not an integral constituent of the lysosomal membrane and provides a source of soluble enzyme free of detergent.