Human glucocerebrosidase catalyses transglucosylation between glucocerebroside and retinol

Abstract

The basal activity of human placental glucocerebrosidase is elevated 16-fold by n-pentanol when assayed usingp-nitrophenyl β-D-glucopyranoside (pNPGlc) as the β-glucosidase substrate. This enhancement of activity is the result of the formation of a transglucosylation product, n-pentyl β-D-glucoside, in rate-determining competition with the hydrolytic reaction. The transglucosylation product accounts for ~ 80% of the reaction product generated in the presence of n-pentanol (0.18 M) when either glucocerebroside or pNPGlc was used as the substrate. This stimulatory effect can be increased an additional 3-fold by the inclusion of phosphatidylserine (20 μg/ml) or sodium taurodeoxycholate (0.3%, w/v) in the incubation medium. In the presence of retinol, glucocerebrosidase also catalyses the synthesis of a novel lipid glucoside, retinyl glucoside, when either glucocerebroside or pNPGlc serves as the substrate. The reaction product was identified as retinyl β-D-glucoside, based on its susceptibility to hydrolysis by almond β-D-glucosidase and the subsequent release of equimolar amounts of retinol and glucose. The rate of retinyl β-glucoside formation is dependent on the concentration of retinol in the incubation medium, reaching saturation at ~ 0.3 mM retinol. Retinyl β-D-glucoside is a substrate for two broad-specificity mammalian β-glucosidases, namely the cytosolic and membrane-associated β-glucosidases of guinea pig liver. However, retinyl β-D-glucoside is not hydrolysed by placental glucocerebrosidase. These data indicate that the glucocerebrosidase-catalysed transfer of glucose from glucocerebroside to natural endogenous lipid alcohols, followed by the action of a broad-specificity β-glucosidase on the transglucosylation product, could provide mammals with an alternative pathway for the breakdown of glucocerebroside to glucose and ceramide.