Hexosaminidase A and

Sandhoff s disease GMj-ganglioside asialo-G -ganglioside, globoside Hexosaminidases A and B... 

OCTAHEDRAL COORDINATION HEXA UNI PING PONG ENZYME MECHANISM HEXOKINASE ATP GTP DEPLETION DEINHIBITION INDUCED FIT HYPOTHESIS 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)... 

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

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... 

Sphingomyelinase Ceramidase (3-Glucocerebrosidase (3-Galactosyl hydrolase (3-Hexosaminidase A (3-Galactosidase a-Galactosidase (3-Hexosaminidases A and B Galactocerebrosidase Arylsulfatase A... 

Tay-Sachs disease is the B-variant of GM2 gangliosidosis due to a-chain deficiency and to the subsequent deficiency of hexosaminidases A and S, buf wifh normal hexosaminidase B. Depending on fhe residual enz)me acfivify of /3-hexosaminidase, fhe onsef of symptoms may occur an)Twhere from late infancy to adulthood and are usually subclassifled into infantile (t) e 1)-, juvenile (type 2)-, chronic-, and adult-onset forms [33]. In type 1, the most common disease with a carrier frequency of 1 in 27 among Ashkenazi Jews [154], patients are normal at birth but then show s)mptoms, such as mild motor weakness, between 3 and 6 months, resulting in hypotonia, poor head control, decreasing attentiveness, and visual symptoms (cherry red... 

The deficiency in hexosaminidase A and B occurring in Sandhoff disease results in accumulation of GM2 and GA2 primarily in the lysosomes of neuronal cells (Fig. 17.3), leading to the formation of membranous cytoplasmic bodies (MCBs) (Tutor, 2004). 

Tay-Sachs disease is caused by the mutation of the alpha subunit of hexosaminidase A gene (HEXA). Deficitated hexosaminidases A and B produce 3 distinct clinical forms of ganglioside GM2 storage disease-Tay-Sachs disease, Sandhoff disease, and juvenile GM2-gangliosidosis. Hexosaminidase-A has a structure comprised of alpha-beta subunits and Tay-Sachs disease is the alpha-minus mutation, whereas Sandhoff disease is a beta-minus mutation (Beutler and Kuhl, 1975 Beutler et al., 1975). Subunit alpha is mapped to chromosome 15 (and beta to chromosome 5). Different levels of residual activities are correlated with the age of clinical onset Tay-Sachs disease, 0.1% of normal hexosaminidase late infantile,... 

Methods for the assay of hexosaminidase A and total hexosaminidase activities in dried blood spots on filter paper offer considerable advantages for screening (Chamoles et al., 2002). The deficient activity of the lysosomal enzymes hexosaminidase A and total hexosaminidase (hexosaminidase A plus B) are usually measured in plasma or extracts of leukocytes. To tubes containing a 3-mm-diameter blood spot, elution liquid and substrate solution were added. After incubation at 37°C, the amount of hydrolyzed product was compared with a calibrator to allow the quantification of enzyme activity. The method was proven reliable even after storage for up to 38 months at room temperature. For total hexosaminidase, the substrate was 4-methyl-umbelliferyl-2-acetamido-... 

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. 

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. 

A third type of mutation also can lead to disease symptoms similar to those of Tay-Sachs disease. Children were identified with Tay-Sachs symptoms, but when both hexosaminidase A and B activities were measured in a test tube, they were normal. This disease, ultimately named Sandhoff activator disease, is caused by a mutation in a protein that is needed to activate hexosaminidase A activity. In the absence of the activator, hexosaminidase A activity is minimal, and GM2 initially accumulates in lysosomes. This mutation has no effect on hexosaminidase B activity. 

Much of the recent progress in our knowledge of the genetics of the sphingolipidoses has come from cell hybridization experiments. Cultured skin fibroblasts have been used to study the isoenzyme abnormalities in Tay-Sachs disease and Sandhoff-Jatzkewitz disease. In the former, only hexosaminidase isoenzyme A is lacking (Okada and O Brien, 1969) while in the latter, both hexosaminidase A and hexosaminidase B are deficient (Sandhoff et al., 1968). The deficiency of hexosaminidase A in these two disorders is due to different mutations and activity can be restored by complementation techniques (e.g. Rattazzi et al, 1976). The studies have also provided support for the idea that hexosaminidase A is composed of two non-identical subunits aP)n and hexosaminidase B is ( )n) (cf. Brady, 1978). Hybridization studies have now identified... 

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.

Variant 0, infantile (Sandhoff disease) /5-Hexosaminidase A and B 5ql3 268800... 

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... 

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