The submicrosomal localization of uridine 5'-diphosphate-glucose dolichyl-phosphate glucosyltransferase and bile acid glucosyltransferase in the human liver

Mutation of beta-glucosidase 2 causes glycolipid storage disease and impaired male fertility

beta-Glucosidase 2 (GBA2) is a resident enzyme of the endoplasmic reticulum thought to play a role in the metabolism of bile acid-glucose conjugates. To gain insight into the biological function of this enzyme and its substrates, we generated mice deficient in GBA2 and found that these animals had normal bile acid metabolism. Knockout males exhibited impaired fertility. Microscopic examination of sperm revealed large round heads (globozoospermia), abnormal acrosomes, and defective mobility. Glycolipids, identified as glucosylceramides by mass spectrometry, accumulated in the testes, brains, and livers of the knockout mice but did not cause obvious neurological symptoms, organomegaly, or a reduction in lifespan. Recombinant GBA2 hydrolyzed glucosylceramide to glucose and ceramide; the same reaction catalyzed by the beta-glucosidase acid 1 (GBA1) defective in subjects with the Gaucher's form of lysosomal storage disease. We conclude that GBA2 is a glucosylceramidase whose loss causes accumulation of glycolipids and an endoplasmic reticulum storage disease.

Stimulation of specific binding of [3H]-progesterone to bovine luteal cell-surface membranes: specificity of digitonin

Non-genomic actions of progesterone have been described in the ovary, and luteal membranes of several species have been shown to possess specific binding sites for [3H]-progesterone. However, binding of radiolabelled progesterone to luteal membranes was demonstrable only in the presence of digitonin. Digitonin is a non-ionic detergent which is thought to act by forming one-to-one complexes with certain sterols. It is also a cardiotonic agent, inhibiting (Na+-K+) ATPase activity by interaction with the extracellular (ouabain/K+) binding site. We therefore investigated which properties of digitonin were responsible for its stimulatory actions on progesterone binding to bovine luteal membranes. A range of compounds with detergent, cardiotonic and or cholesterol-complexing activities were tested for their effects on [3H]-progesterone binding to bovine luteal membrane fractions, and on haemolysis of rat erythrocytes. Stimulation of progesterone binding to luteal membranes was highly specific for digitonin, and a number of ionic and non-ionic detergents, cardenolides, saponins and cholesterol-complexing reagents tested failed either to stimulate [3H]-progesterone binding to bovine luteal membranes in the absence of digitonin, or to inhibit binding specifically in the presence of digitonin. When digitonin was first reacted with excess cholesterol or pregnenolone to form the respective digitonides, stimulatory activity was greatly reduced, suggesting that the ability of digitonin to interact with (an) endogenous steroid(s) may be important in its action. High performance liquid chromatography (HPLC)-mass spectrometry of commercially available digitonin preparations indicated the presence of numerous minor impurities in most commercial digitonin preparations. Three major UV-absorbing peaks were isolated and characterised by mass spectrometry: all stimulated progesterone binding to bovine luteal membrane receptors in a dose-dependent manner, though to differing extents. Our data suggest that the unique action of digitonin on luteal membrane progesterone receptors is not related to its detergent or cardiotonic properties, but appears to be related to its ability to complex with membrane sterols.

Purification and characterization of a microsomal bile acid beta-glucosidase from human liver

A human liver microsomal beta-glucosidase has been purified to apparent homogeneity in sodium dodecyl sulfate-polyacrylamide gel electrophoresis where a single protein band of Mr 100,000 was obtained under reducing conditions. The enzyme was enriched about 73, 000-fold over starting microsomal membranes by polyethylene glycol fractionation, anion exchange chromatographies on DEAE-Trisacryl, and Mono Q followed by affinity chromatography on N-(9-carboxynonyl)-1-deoxynojirimycin-AH-Sepharose 4B. The purified enzyme had a pH optimum between 5.0 and 6.4, was activated by divalent metal ions, and required phospholipids for exhibition of activity. The enzyme catalyzed the hydrolysis of 3beta-D-glucosido-lithocholic and 3beta-D-glucosido-chenodeoxycholic acids with high affinity (Km, 1.7 and 6.2 microM, respectively) and of the beta-D-glucoside (Km, 210 microM) and the beta-D-galactoside of 4-methylumbelliferone. The ratio of relative reaction rates for these substrates was about 6:3:11:1. No activity was detectable toward 6beta-D-glucosido-hyodeoxycholic acid, glucocerebroside, and the following glycosides of 4-methylumbelliferone: alpha-D-glucoside, alpha-L-arabinoside, beta-D-fucoside or beta-D-xyloside. Immunoinhibition and immunoprecipitation studies using antibodies prepared against lysosomal glucocerebrosidase showed no cross-reactivity with microsomal beta-glucosidase suggesting that these two enzymes are antigenically unrelated.

The subcellular localization of a bile acid glucosyltransferase in rat liver

Formation of bile acid glucosides occurs in rat liver homogenate with a specific enzyme activity of 0.014 +/- 0.001 nmol per min per mg protein. Subcellular fractionation of rat liver by differential centrifugation revealed an enrichment of bile acid glucosyltransferase activity both in the mitochondrial-lysosomal fraction and in microsomes with a recovery of 38.8 +/- 4.6% and 37.7 +/- 1.7%, respectively, of enzyme activity in the homogenate. Subfractionation of the mitochondrial-lysosomal fraction after treatment of rats with Triton WR 1339 showed an almost exclusive association of bile acid glucosyltransferase activity with purified lysosomes ("tritosomes"). After subfractionation of microsomes by analytical gradients, bile acid glucosyltransferase was bimodally distributed with peaks at modal densities of 1.09 g/cm3 and 1.16 g/cm3, respectively. If microsomes were pretreated with pyrophosphate, a membrane perturbant known to strip ribosomes, only the peak of bile acid glucosyltransferase at higher density (1.16 g/cm3) and UDP-glucuronosyltransferase (marker of endoplasmic reticulum) shifted to a similar lower equilibrium density. Both enzymes were unaffected in their distribution by pretreatment of microsomes with digitonin. In contrast, markers of plasma membranes (5'-nucleotidase) and the Golgi-complex (galactosyltransferase) shifted to higher equilibrium densities after digitonin treatment, but were unaltered in their distribution after pyrophosphate. Bile acid glucosyltransferase activity in the lower density range with a peak at 1.09 g/cm3 did not show any association with the density distributions of known marker enzymes. In purified microsomal fractions obtained by preparative gradients, bile acid glucosyltransferase activity was enriched in enzyme activity by 1.4-fold in rough and by 2.3-fold in smooth endoplasmic reticulum, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)