Interconversion of the A and B forms of ceramide trihexosidase from human plasma

The human plasma α-galactosidases which specifically hydrolyze galactosyl-(α1→4)galactosyl(β1→ 4)glucosylceramide consist of an A group with optimal enzymatic activity at pH 5.4, and a B group, which is characterized by optimal activity at pH 7.2. The relationship between the A and B groups of these α-galactosidases (ceramide trihexosidases) has been investigated with regard to their sialic acid content. Partial neuraminidase treatment of the most acidic (A-1) form of ceramide trihexosidase yields a complex mixture of 14 enzymatically active proteins separable by isoelectric focusing. Exposure to neuraminidase for a longer time causes an almost complete conversion of the A-1 form to a protein which has the same electrophoretic properties as the least acidic (B-V) form. Conversely, a crude kidney sialyltransferase preparation can be used to incorporate either CMP[1- 14C]sialic acid or UDP- N-acetyl[1- 14C]glucosamine into the B-V form of the enzyme. Sialyltransferase treatment causes the formation of a complex mixture of enzymatically active proteins, one of which has the same electrophoretic characteristics as the A-1 and A-2 forms of ceramide trihexosidase. On the basis of these studies it is suggested that the multiple forms of plasma ceramide trihexosidase are glycoproteins which differ primarily in their sialic acid content.