GM2 gangliosidosis B1 variant: biochemical and molecular characterization of hexosaminidase A.

Abstract

The biochemical properties of hexosaminidase A (HexA) and the coding sequence of the alpha-subunit were examined in a patient of Syrian ancestry with the B1 form of Tay-Sachs disease (TSD). The biochemical characteristics of the variant HexA suggest that both active sites are affected by the mutation(s). Kinetic studies with the beta-subunit specific substrate, 4-methylumbelliferyl-beta-D-N-acetylglucosamine (MUG), revealed a significant difference between the Km values. of normal and variant HexA, while no difference was found when the sulfated substrate MUG-6-sulfate (MUGS), which is specific for the alpha-subunit active site, was used. The Vmax values for both substrates were significantly lower in extracts from B1 variant cells than in control extracts, implying a reduced enzyme level in the variant cells. A noncompetitive inhibitor of the reaction with MUGS, N-acetylglucosamine (NAG), induced a significant inhibition (30%) in the mutant cells only. When MUG was used as substrate, variant HexA was found to be more heat stable (T50 = 170 min) than normal HexA (T50 = 65 min). Furthermore, the mutant cell preparation differed from control in the relation between Hex thermosensitivity and protein concentration in the reaction. Two new mutations were identified in exon 5 of the HexA gene: a C496 to G transversion, which produced an Arg166 -->Gly alteration and a deletion of C498 which generated a shift in the reading frame. The patient was a heterozygote for both mutations even though her parents are first cousins. There is no evidence as yet which of these mutations accounts for the B1 phenotype.

• Authors: L Peleg, F Meltzer, M Karpati, B Goldman
• Journal: Biochemical and molecular medicine (Biochem Mol Med) Vol. 54 Issue 2 Pg. 126-32 (Apr 1995) ISSN: 1077-3150 [Print] United States
• PMID: 8581357 (Publication Type: Journal Article)

Chemical References

• Hexosaminidase A
• beta-N-Acetylhexosaminidases

Topics

• Base Sequence
• Cells, Cultured
• Exons
• Female
• Fibroblasts (cytology, enzymology)
• Hexosaminidase A
• Humans
• Kinetics
• Male
• Molecular Sequence Data
• Mutation
• Polymorphism, Single-Stranded Conformational
• Tay-Sachs Disease (enzymology, etiology, genetics)
• beta-N-Acetylhexosaminidases (antagonists & inhibitors, chemistry, physiology)