Sulfatide activator

Furst W, Machleidt W, Sandhoff K (1988) The precursor of sulfatide activator protein is processed to three different proteins. Biol Chem Hoppe-Seyler 369 317-328... 

The activator of cerebroside sulfatase (or sulfatide activator ) was purified by Fischer and Jatzkewitz (1975) from human liver and identified as a water-soluble glycoprotein with an isoelectric point at pH 4.3 and a molecular weight of approximately 22,000 Daltons. From kinetic and binding experiments and from the fact that this cofactor stimulated only the degradation of lipid substrates but not of artificial water-soluble ones, these authors concluded that the cofactor serves to solubilize the lipid by binding to it and extracting it from the membrane (or micelle). The resulting activator/lipid complex was assumed to be the true substrate of the enzymic reaction (Fischer and Jatzkewitz, 1977, 1978). 

Those activator proteins that promote the hydrolysis of glycolipid substrates by water-soluble hydrolases, namely the Gm2 activator and the sulfatide activator, seem to act by similar mechanisms they bind the glycolipid and extract it from the membrane to form a water-soluble complex that is the true substrate for the enzyme. Accordingly, the reaction rate depends on the concentration of the activator-lipid complex, with Michaelis-Menten-type saturation kinetics (Conzelmann and Sandhoff, 1979). Such a saturation curve is at no point exactly linear, but for practical purposes sufficient linearity can be assumed at activator concentrations below the value of the enzyme for the respective activator-lipid complex as substrate. 

During this procedure the Gm2 activator is separated from the sulfatide activator by the ion-exchange chromatography step and can be further purified as described earlier. 

The first activator protein was discovered in 1964 as a protein required for the hydrolytic degradation of sulfatides by lysosomal arylsulfatase A [18]. This sulfatide-activator or SAP-B (saposin B), binds GSLs, but with broader specificity than the GM2 activator. In vitro it behaves similarly to the GM2 activator in some aspects, i.e. it can present GSLs to water-soluble enzymes as substrates [19). 

In case of a deficiency of arylsulfatase A, at least one other sulfatase should be measured to exclude multiple sulfatase deficiency (see Chap. 4.1 for the assays of arylsulfatase and other sulfatases). Sulfatide excretion in urine should be measured (see assay below) and/or mutation analysis should to performed to confirm the diagnosis, especially if the clinical symptoms are atypical and in order to exclude a pseudodeficiency of arylsulfatase A. The enzyme should always be measured in the parents to check for the presence of compound heterozygosity of a metachromatic leukodystrophy and a pseudodeficiency allele. This is very important for the interpretation of the results of arylsulfatase A assays, especially when performed in asymptomatic or presymptomatic siblings or in the context of a prenatal diagnosis. Sulfatide should also be measured in case a normal arylsulfatase A activity is found in a patient with symptoms characteristic of (juvenile) metachromatic leukodystrophy. Increased urinary sulfatide excretion is indicative of an activator protein/saposin deficiency (Fig. 4.4.1). 

A dramatic increase (approx. 50-fold) of the activity of the sulfotransferase involved in the biosynthesis of the SGG also occurred when spermatocytes first began to appear in rat testis (19) the rise in the activity of this enzyme preceded by several days a marked rise in the amount of the SGG. Studies on pre-puber-tal human testis (which is temporarily blocked in spermatogenesis at a stage prior to the appearance of primary spermatocytes) have shown that neither SGG nor GG is present (15,26). Similarly, the testis of the pre-pubertal fowl also lacks sulfogalactosylceramide, the sulfatide found in mature fowl testis (26). All of these find-... 

The vitamin also activates serine palmitoyltransferase, the first enzyme of phosphosphingolipid synthesis, and in bacteria it can, together with inorganic phosphate, replace part of the ATP requirement of galactocerebroside sulfo-transferase (Tsaioun, 1999). In animals, lipid sulfatides are decreased in vitamin K deficiency and increased with higher intakes (Sundaram et al., 1996). 

Previous work, which has not been revisited, showed that sulfatides are necessary for the optimal function of enzymes such as sodium-potassium-dependent ATPase, and the sulfatide content seems to be directly related to the activity of the enzyme (Karlsson et al., 1974). Sulfatides may also be involved in the functioning of certain opiate receptors (Craves et al., 1980) and in chloride transport systems (Zalc et al., 1978). Implantation in the spinal cord of a hybridoma secreting specific antisulfatide antibodies has been shown to cause demyelination of the CNS in the rat (Rosenbluth et al., 2003). Antisulfatide antibodies have been found in HIV... 

Fujibayashi, S., and Wenger, D. A., Biosynthesis of the sulfatide/GM activator protein (SAP-1) in control and mutant cultured skin fibroblasts. Biochem. Biophys. Acta 873, 534-562 (1986). 

Inui, K., Emmett, M., and Wenger, D. A., Immunological evidence for deficiency in an activator protein for sulfatide sulfatase in a variant form of metachromatic leukodystrophy. Proc. Natl. Acad. Sci. U.S.A. 80, 3074-3077 (1983). 

Wenger, D. A., and Inui, K., Studies on the sphingolipid activator protein for the enzymatic hydrolyses of CMj ganglioside and sulfatide. In Molecular Basis of Lysosomal Storage Disorders (J. A. Barranger and R. O. Brady, eds.), pp. 61-78. Academic Press, Orlando, 1984. 

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