66] Activator proteins for lysosomal glycolipid hydrolysis

Abstract

Publisher Summary The final degradation of glycosphingolipids occurs in the digestive intracellular organelles known as lysosomes and is accomplished by the sequential action of specific hydrolases. Some of these enzymes appear to be membrane associated and interacting with their lycolipid substrates directly. Others, mainly those acting also on water-soluble substrates, are water soluble and cannot attack their lipid substrates directly. Instead, this interaction has to be mediated by some small nonenzymatic proteins, which are called activator proteins. The physiological significance of activator proteins is demonstrated by two fatal lipid storage diseases that are caused by deficiencies of activator proteins rather than of the degrading enzymes. Those activator proteins that promote the hydrolysis of glycolipid substrates by water-soluble hydrolases, which is, the G M2 activator and the sulfatide/G M1 activator, seem to act by similar mechanisms: they bind the glycolipid and extract it from the membrane to form a water-soluble complex, which is the true substrate for the enzyme. The chapter discusses the assay method of sulfatide/G M1 activator protein with Ganglioside G Ml /β-Galactosidase, Sulfatide/Arylsulfatase A, and Globotriaosylceramide/a-Galactosidase A. There is also a discussion about G M2 activator. Its measurement is based on its ability to accelerate the hydrolysis of ganglioside G M2 by β-hexosaminidase A. The procedure for radioactive labeling of the terminal N-acetylgalactosamine moiety with galactose oxidase/NaB 3 H 4 85 is essentially the same as for ganglioside G M1 . The chapter discusses Cofactors for Glucosylceramide β-Glucosidase and Galactosylceramide β-Galactosidase. There is also a description of the purification procedures of the activator proteins.