Glycosphingolipid β-Galactosidases

Specific glycosphingolipid β-galactosidases in normal human liver were investigated using radioactively labeled galactosylceramide, lactosylceramide, Gm1-ganglioside, and asialo Gm1-ganglioside as substrates. Chloride ion was required for full stimulation and stabilization of these reactions. Taurocholate was the most effective stimulator, except for Gm1-ganglioside β-galactosidase which did not appear to be affected by any of the detergents tested. Electrofocusing of the 10,000 x g supernatant of sonicated, frozen-thawed water homogenates gave three consistent peaks of nonspecific 4-methylumbelliferyl β-galactosidase activities. Their isoelectric points were pH 4.1 to 4.2 (α), 4.5 to 4.6 (β), and 4.8 to 4.9 (γ). All of the three peaks contained activities of lactosylceramide and asialo Gm1-ganglioside β-galactosidases, but the activities of galactosylceramide and Gm1-ganglioside β-galactosidases were present only in the β and γ peaks. Gel filtration with Sephadex G-200 separated activities of 4-methylumbelliferyl β-galactosidase into three peaks. When each of the gel filtration peaks was subjected to electrofocusing, the second gel filtration peak corresponded to the β peak, and the third peak to the γ peak, respectively. The first gel filtration peak, however, gave inconsistent results in electrofocusing, always giving a single peak but with varying isoelectric points. The primary purpose of this study was to establish the optimal assay conditions for these enzymes in whole tissues and to obtain a reproducible fractionation procedure, without sacrificing the recovery of total activities. These were the criteria necessary for the subsequent studies of disorders in which specific glycosphingolipid β-galactosidases are genetically deficient.