Variable expression of leukocyte cytosolic broad-specificity beta-glucosidase activity

The cytosolic beta-glucosidase activity that is found in a variety of mammalian tissues has no clearly defined function. In vitro assay conditions under which the broad-specificity beta-glucosidase hydrolyzes glucocerebroside at a significant rate have not been described. Nonetheless, it has been suggested that this enzyme might play an accessory role with lysosomal glucocerebrosidase in catalyzing the hydrolysis of glucosylceramide. However, this hypothesis would require that activity of both enzymes be low in severe cases of Gaucher disease in which there are pathological accumulations of glucosylceramide in one or more of the affected organs, i.e. spleen, liver and bone marrow. Information is lacking regarding the normal range of cytosolic beta-glucosidase activity in humans. p-Nitrophenyl-beta-D-mannoside was found to be a potent inhibitor (Ki = 0.068 mM) of cytosolic beta-glucosidase. In parallel studies, the activity of glucocerebrosidase was found to be minimally affected by p-nitrophenyl-beta-D-mannoside at concentrations as high as 2.5 mM. This information was used to design an assay system that would allow us to estimate glucocerebrosidase and cytosolic beta-glucosidase activities in extracts of human leukocytes. Average cytosolic beta-glucosidase activity with 4-heptyl-umbelliferyl-beta-D-glucoside as a substrate was 8.8 nmol/h/mg protein in leukocytes from 356 subjects (range, 0.2-28). Average leukocyte glucocerebrosidase specific activity was 16 nmol/h/mg protein (range 5.3-45.7). No correlation was observed between cytosolic beta-glucosidase and glucocerebrosidase activity for control and Gaucher heterozygote populations (r = 0.19 and 0.25, respectively). The wide range of leukocyte cytosolic beta-glucosidase activity in individuals tested in this study indicates that a substantial proportion of the population may lack sufficient cytosolic beta-glucosidase activity to assist a defective lysosomal glucocerebrosidase in hydrolyzing glucosylceramide.