P-Hexosaminidase A and

The best known and the commonest sphingolipi-dosis is Tay-Sachs disease.366-368 Several hundred cases have been reported since it was first described in 1881. A terrible disease, it is accompanied by mental deterioration, blindness, paralysis, dementia, and death by the age of three. About 15 children a year are born in North America with this condition, and the world figure must be 5-7 times this. The defect is in the a subunit of the (3-hexosaminidase A (point 7 in Fig. 20-10)366,366a with accumulation of ganglioside GM2- Somewhat less severe forms of the disease are caused by different mutations in the same gene369 or in a protein activator. Sandhoff disease, which resembles Tay-Sachs disease, is caused by a defect in the (3 subunit, which is present in both P-hexosaminidases A and B.368 Mutant "knockout" mice that produce only ganglioside GM3 as the major ganglioside in their central nervous system die suddenly from seizures if they hear a loud sound. This provides further evidence of the essential nature of these components of nerve membranes.3693... 

The specific interaction between p-hexosaminidase A and the Gm2 activator has also been taken into account in the development of assay systems for ganglioside Gm2 hydrolysis by extracts of cultured skin fibroblasts (Erzberger et al., 1980 Conzelmann et al., 1983). With such assays the residual activities in cells from patients with different variants of Gm2 gangliosidosis could be determined very precisely (Conzelmann et al., 1983). 

Gi/mol), 2.5 p,g bovine serum albumin, 100 mU P-hexosaminidase A, and up to 25 p.1 of the suitably diluted activator sample in a total volume of 40 (il mM citrate buffer, pH 4.0, and is incubated for 1 -4 h at 37°C. Samples are then transferred to an ice bath and loaded onto 1 ml columns of DEAE-cellulose (in Pasteur pip>ettes) that have been washed with distilled water. Liberated [ H]A-acetylgalactosamine is eluted with 2 x 1 ml of a 1 mM aq A-acetylgalactosamine solution. The combined effluents are collected in scintillation vials, and after addition of 10 ml scintillation fluid, their radioactivity is measured. Blanks run with water instead of activator solution are subtracted. 

Sphingomyelinase Ceramidase P-Glucocerebrosidase P-Galactosyl hydrolase P-Hexosaminidase A P-Galactosidase a-Galactosidase P-Hexosaminidases A and B Galactocerebrosidase Arylsulfatase A... 

Figure 8. Model for the lysosomal degradation of a membrane-bound glycosphingoUpid by water-soluble lysosomal exohydrolases and membrane-active activator proteins. This example illustrates the degradation of ganglioside GM2 by P-hexosaminidase A and GM2-activator protein. BMP, bis (monoacylglycero)phosphate.

Leinekugel, P, Michel, S, Conzelmann, E and Sandhoff, K (1992) Quantitative correlation between the residual activity of P-hexosaminidase A and arylsulfatase A and the severity of the resulting lysosomal storage disease. Hum Genet, 88, 513-523. 

Sandhoff disease Panethnic but otherwise virtually indistinguishable from Tay-Sachs disease GM2 ganglioside, asialo-GM2 ganglioside, globoside Glycoprotein with terminal P-hexosamine P-Hexosaminidase A and B... 

GM2-gangliosidosis, O-variant, Sandhoff disease 268800 -Hexosaminidase A and (EC 3.2.1.52) L, F, P, D GM2-ganglioside, GA2 (asialio-GM2)-ganglio-side, globoside U (oligosaccharides)... 

In our laboratory an authentic in vitro cellular model for Sandhoff disease has been generated upon treatment of RAW264.7 murine macrophages with an inhibitor of p-hexosaminidase (Boomkamp and Butters, unpublished data). High performance liquid chromatography (HPLC) analyses of extracted GSL oligosaccharides show that GA2 predominantly accumulates in inhibitor-treated RAW cells as opposed to a minor but significant increase in GM2 levels (Fig. 17.7). This difference in elevation is possibly due to the... 

Mutation of the HexB gene, and production of a defective p subunit, leads to inactivation of both hexosaminidase A and B activity. Such a mutation leads to Sandhoff disease. Both activities are lost because both activities require a functional P subunit. The clinical course of this disease is similar to Tay-Sachs but with an accelerated timetable because of the initial accumulation of both GM2 and globoside in the lysosomes. 

Granberg, M., Fowler, C. J., and Jacobsson, S. O. P. (2001). Effects of the caimabimimetic fatty acid derivatives 2-arachidonoylglycerol, anandamide, palmitoylethanolamide and methanandamide upon IgE-dependent antigen-induced p-hexosaminidase, serotonin and TNF a release from rat RBL-2H3 basophilic leukaemia cells. Naunyn Schmiedebergs Arch. Pharmacol. 364, (y6-l i. 

In addition to an enzyme, the presence of the GM2-activator protein in vivo or of a detergent in vitro is required for the digestion of ganglioside GM2. Mutations in the gene of the P-hexosaminidase a-chain in human patients leads to Tay-Sachs disease, and mutations in the P-chain lead to Sandhoff disease. Together with GM2-activator protein deficiency, these diseases belong to the GM2-gangUosidoses. 

Werth, N, Schnette, CG, Wdkening, G, Lemm, T and Sandhoff, K (2001) Degradation of membrane-bonnd ganglioside GM2 by P-hexosaminidase A. J Biol Chem, 276, 12685-12690. 

Figure 2. Model for the GM2-activator stimulated degradation of ganglioside GM2 by human P-hexosaminidase A (modified from [12]). Water-soluble P-hexosaminidase A does not degrade membrane-bound ganglioside GM2, whieh has a short earbohydrate chain, in the absence of GM2-activator or appropriate detergents. The GM2-activator binds one molecule of ganglioside GM2 and lifts it out of the membrane. The activator-lipid complex can be recognized by water-soluble P-hexosaminidase A which cleaves the lipid substrate.

GM2 molecule to yield ganglioside GM3, and releases the activator-lipid complex. The activator in turn may release GM3 into the membrane and bind another GM2 molecule. Formation of the ternary complex presumably involves also a protein-protein interaction between the GM2 activator and the glycosidase, p-hexosaminidase A [17]. 

Ohno, K., and Suzuki, K., 1988b, A splicing defect due to an exon-intron junctional mutation results in abnormal P-hexosaminidase a chain mRNAs in Ashkenazi Jewish patients with Tay-Sachs disease, Biochem. Biophys. Res. Commun. 153 463-469. 

Proia, R. L., and Soravia, E., 1987, Organization of the gene encoding the human p-hexosaminidase a chain, J. Biol. Chem. 262 5677-5681. 

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