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  • Title: Molecular genetics of GM1 beta-galactosidase.
    Author: O'Brien JS.
    Journal: Clin Genet; 1975 Nov; 8(5):303-13. PubMed ID: 812620.
    Abstract:
    The molecular genetics of GM1 beta-galactosidase is reviewed. This enzyme exists in two forms, A and B. Form A is monomeric with a molecular weight of 72,000 and appears to be coded by a single autosomal locus. Form B is polymeric and cross-reacts with anti-A antibodies; it is coded wholly or in part by the same locus that codes for A. The simultaneous loss of A and B in GM1 gangliosidosis is explained. None of the other beta-galactosidases, including neutral beta-galactosidase, ceramide lactoside beta-galactosidase or cerebroside beta-galactosidase cross-react with anti-A antibodies, demonstrating that they are coded by loci separate from A. GM1 beta-galactosidase A is heterocatalytic, cleaving beta-D-galactose from ganglioside GM1, lactose, N-acetyllactosamine, and galactose-containing glycoproteins such as asialofetuin, red cell stromal glycoproteins and keratan sulfate. The pleotropic effects of a single mutation affecting the locus for beta-galactosidase A can be explained by a one gene:one polypeptide:many substrates model. Phenotypic variability among beta-galactosidase A mutants may result from better residual activity of the mutant enzyme for one substrate than for another. Patients with normal intelligence and severe bony deformities, who are homozygous for a mutation affecting the enzyme, illustrate this point. Thus far all human mutants for GM1 beta-galactosidase studied are structural mutants, synthesizing nearly normal quantities of mutant enzyme; one is a proven Km mutant, the others are very likely so.
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