Expression of cytosolic ?-glucosidase in guinea pig liver cells

Abstract

The cytosolic ␤-glucosidase of mammalian liver has been implicated in the metabolic transformation of plant glycosides, such as vicine and amygdalin, which are associated with the development of toxic syndromes. We investigated which cell types express cytosolic ␤-glucosidase in guinea pig liver, and characterized the contribution of this enzyme to the hydrolysis of aromatic glucosides in cultured cells and in tissue slices. Cytosolic ␤-glucosidase was expressed in hepatocytes and not in Kupffer or endothelial cells as determined by enzyme-specific activity and Western blots of liver cell extracts. Intracellular ␤-glucosidase activity was visualized using the fluorescent ␤-glucosidase substrate, resorufin ␤-D-glucoside, and shown to be caused by the cytosolic ␤-glucosidase using the inhibitors, conduritol ␤-epoxide and dinitrophenol-2-deoxy-2-fluoro-␤-D-glucopyranoside (DNP2FGlc). Staining of fresh liver slices with resorufin ␤-glucoside revealed that cytosolic ␤-glucosidase is expressed in all hepatocytes, with no significant portalcentral gradient. These data indicate that cytosolic ␤-glucosidase is a hepatocyte-specific enzyme, and support the hypothesis that cytosolic ␤-glucosidase in the liver functions to hydrolyze small glucosides absorbed by the intestine. Furthermore, toxic injury to cultured hepatocytes by CCl 4 resulted in release of cytosolic ␤-glucosidase in parallel with the hepatocyte marker enzymes alanine transaminase and lactate dehydrogenase. This suggests that acute increases in serum levels of cytosolic ␤-glucosidase in animal models of liver injury may reflect direct injury of hepatocytes. (HEPATOLOGY 1998;28:156-163.)A principal physiological function of the liver is to dispose of xenobiotic compounds absorbed from the intestinal tract. The detoxification and excretion pathways for numerous compounds are classified into two stages. Phase I reactions involve either oxidation or reduction reactions, and are typically catalyzed by cytochrome P450 enzymes. 1 Phase II reactions then conjugate the modified compound, typically with glucuronic acid, sulfate, or glutathione, before excretion of the conjugate into the bile. 2 In plants, however, a variety of physiologically relevant compounds are produced and stored as O-linked ␤-D-glucosides. 3-5 Although the metabolic disposal of such glucosides by mammals is largely uncharacterized, it is likely that removal of the glucose moiety occurs at some point before ultimate excretion. Furthermore, the aglycone moieties of ␤-D-glucosides found in dietary plants have been implicated as the etiologic agents in acute cyanide intoxication resulting from ingestion of cassava, 6 apricot seeds, 7 and administration of amygdalin (laetrile), 8 as well as neurodegenerative disorders such as western pacific amyotrophic lateral sclerosis/parkinsonism-dementia. 6 The cytosolic ␤-glucosidase of mammalian liver has been characterized and studied by several groups. 9 The enzyme has been described in the livers of humans, 10 cattle, 11 rabbits, 12 and guinea pigs. 13 Cyto...

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