Hydrolysis of ceramide trihexoside by a specific -galactosidase from human liver.

1. Partially purified ceramide trihexoside alpha-galactosidase from human liver was studied by using ceramide trihexoside specifically tritiated in the terminal galactose. 2. The hydrolysis of ceramide trihexoside was absolutely dependent on a mixture of sodium taurocholate and Triton X-100 and was markedly inhibited by human serum albumin and by NaCl. 3. The Lineweaver-Burk plot for ceramide trihexoside hydrolysis was upward curving. Ceramide lactoside inhibited hydrolysis of all concentrations of ceramide trihexoside. Ceramide digalactoside stimulated hydrolysis of low concentrations of ceramide trihexoside, but inhibited hydrolysis of high concentrations of the lipid. 4. alpha-Galactosidase activity assayed with the synthetic substrate 4-methylumbelliferyl alpha-d-galactopyranoside fractionated together with activity assayed with the natural substrate ceramide trihexoside. Both activities had identical heat-inactivation kinetics. 5. Characteristics of the hydrolysis of the synthetic substrate differed considerably from those of the natural substrate, including pH optimum, shape of the Lineweaver-Burk plot, and differential effects of inhibitors and activators. Mutual inhibition of hydrolysis between the synthetic and natural substrates was predominantly non-competitive. 6. These results are discussed in the light of special problems involved in the hydrolysis of lipids in an aqueous milieu.