Saposin B is involved in the hydrolysis of
sulfatides,
GM1 ganglioside,
globotriaosylceramide, and several other
sphingolipids and glycerolipids by lysosomal
hydrolases.
Saposin B is one of four small
glycoproteins (
saposins) derived from prosaposin. The
carbohydrate chain of
saposin B was removed and deglycosylated
saposin B was characterized and compared with native
saposin B. Deglycosylated
saposin B stimulated the enzymatic hydrolysis of
ganglioside GM1 by
acid beta-galactosidase and
sulfatide by
arylsulfatase A to the same extent as native
saposin B. In addition deglycosylated
saposin B bound
sulfatide and
GM1 ganglioside identical to native
saposin B. The stability of native
saposin B to proteolytic digestion was unchanged by deglycosylation. Neither native
saposin B nor deglycosylated
saposin B were hydrolyzed by
trypsin,
endoproteinase Glu-C (V-8),
chymotrypsin, or a mixture of
acid proteases isolated from human testis. Unlike its effect on metabolic stability, the
carbohydrate chain appears to affect folding of
saposin B. When native and deglycosylated
saposin B were reduced under denaturing conditions and refolded under identical conditions examination of the refolded products indicated that each
protein was refolded in a qualitatively different way. A human mutation in
saposin B-deficient
metachromatic leukodystrophy, in which its glycosylation site is eliminated, has been reported. Our observations suggest that instability of the mutated
saposin B is not due to the absence of a protective effect of the
carbohydrate chain on proteolysis, but is likely due to aberrant folding resulting from the absence of a
carbohydrate chain.