Substrate Specificities of the Two Genetically Distinct Human Brain Beta-galactosidases

The two human brain beta-galactosidases were solubilized and fractionated by Sephadex G-200 gel filtration, free from each other. Substrate specificities of the two enzymes were examined for galactosylceramide, lactosyl-[N-stearoyl]ceramide, lactosyl-[N-lignoceroyl]ceramide, galactosyl-N-acetylgalactosaminyl-[N-stearoyl]ceramide, lactosyl-[N-lignoceroyl]ceramide, galactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl]galactosyl-glucosylceramide (GMI-ganglioside), galactosyl-N-acetylgalactosaminyl-galactosyl-glucosylceramide (asialo GM1-ganglioside), and 4-methylumbelliferyl beta-galactoside. Under appropriately optimized conditions, either of the two beta-galactosidases could hydrolyze all of the substrates, although with widely varying rates. Relative specific activities of galactosylceramide beta-galactosidase toward galactosylceramide, lactosyl-[N-steroyl]ceramide, lactosyl-[N-lignoceroyl]ceramide. GM1-ganglioside, asialo GM1-ganglioside, and 4-methylumbelliferyl beta-galactoside were 100, 510, 250, 39, 41 and 120, respectively. Relative specific activities of GM1-ganglioside beta-galactosidase toward the same series of the substrates were 0.3, 78, 19, 100, 150 and 240; However, the optimal assay conditions for any given natural substrate were sufficiently different for each beta-galactosidase so that diagnostic assays for the two genetic diseases due to beta-galactosidase deficiencies could be carried out in whole tissues. Since the relative distribution of the two enzymes vary greatly in different tissues, contributions by the two enzymes to degradation of the natural glycosphingolipids in vivo may well vary in different organs. These findings may have an important bearing on the biochemical pathogenesis of these genetic disorders.