A benign deficiency of typeB beta-galactosidase in human liver

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.

Inactivation of GM1-ganglioside beta-galactosidase by a specific inhibitor: a model for ganglioside storage disease

This study was designed to establish an in vitro model with biochemical and morphological similarities to the human neurodegenerative disease GM1 gangliosidosis. Utilizing a specific inactivator of the lysosomal enzyme GM1-ganglioside beta-galactosidase (beta-D-galactopyranosylmethyl-p-nitrophenyltriazene [beta-GalMNT]) and neuroblastoma X glioma hybrid cells (NG108-15), we suppressed beta-galactosidase activity for up to 72 hours. Coincidental with suppression of this enzyme to levels less than 1% of control, we found up to a nine-fold accumulation of its substrate, the GM1-ganglioside, and the ultrastructural appearance of membranous cytoplasmic bodies. beta-GalMNT treatment suppressed growth but had little effect on the specific activity of choline acetyltransferase, lactate dehydrogenase, or other lysosomal enzymes including galactosylceramidase. This model should permit studies of the neurophysiological effects of increased ganglioside accumulation and their reversibility.

Multiple carbohydrate-cleaving specificities in human acidic and neutral glycosidases

The common identity of human acidic beta-D-glucosidase (beta-D-glucoside glucohydrolase, EC 3.2.1.21) and beta-D-xylosidase (1,4-beta-D-xylan xylohydrolase, EC 3.2.1.37) as one enzyme and that of acidic beta-D-galactosidase (beta-D-galactoside galactohydrolase, EC 3.2.1.23), beta-D-fucosidase (no allotted EC number) and alpha-L-arabinosidase (alpha-L-arabinofuranoside arabinohydrolase, EC 3.2.1.55) as another enzyme is indicated by similar binding patterns of glycosidase activities of each enzyme to various lectins. by similar ratios between their intra- and extracellular levels in normal and I-cell fibroblasts and by their deficiencies in liver tissues from patients with Gaucher disease and GM1 gangliosidosis, respectively. A third enzyme, neutral beta-D-galactosidase, purified to homogeneity from human liver has been shown to possess all these five glycosidase activities at neutral pH. These neutral enzymic activities were not bound by any of the lectins examined and found to be reduced in liver and spleen of a patient with neutral beta-D-galactosidase deficiency. An additional form of beta-D-xylosidase with optimal activity at pH 7.4 was bound by the fucose-binding lectin from Ulex eurpaeus while no binding was observed for the acidic (pH 4.8) and neutral (pH 7.0) beta-D-xylosidase activities of the multiple glycosidase enzymes.

Biochemistry and genetics of gangliosidoses

The gangliosidoses comprise an-ever increasing number of biochemically and phenotypically variant diseases. In most of them an autosomal recessive inherited deficiency of a lysosomal hydrolase results in the fatal accumulation of glucolipids (predominantly in the nervous tissue) and of oligosaccharides. The structure, substrate specificity, immunological properties of and genetic studies on the relevant glycosidases, ganglioside GM1 beta-galactosidase and beta-hexosaminidase isoenzymes, are reviewed in this paper. Contrary to general expectation, only a poor correlation is observed between the severity of the disease and residual activity of the defective enzyme when measured with synthetic or natural substrates in the presence of detergents. For the understanding of variant diseases and for their pre- and postnatal diagnosis, the necessity of studying the substrate specificity of normal and mutated enzymes under conditions similar to the in vivo situation, e.g., with natural substrates in the presence of appropriate activator proteins, is stressed. The possibility that detergents may have adverse affects on the substrate specificity of the enzymes is discussed for the beta-hexosaminidases. The significance of activator proteins for the proper interaction of lipid substrates and water-soluble hydrolases is illustrated by the fatal glycolipid storage resulting from an activator protein deficiency in the AB variant of GM2-gangliosidosis. Recent somatic complementation studies have revealed the existence of a presumably post-translational modification factor necessary for the expression of ganglioside GM1 beta-galactosidase activity. This factor is deficient in a group of variants of GM1-glangliosidosis. Among the possible reasons for the variability of enzyme activity levels in heterozygotes and patients, allelic mutations, formation of hybrid enzymes, and the existence of patients as compound heterozygotes are discussed. All these may result in the production of mutant enzymes with an altered specificity for a variety of natural substrates.