Human Kidney α-Galactosidase: Multiplicity and Enzyme Activities for Ceramide Trihexoside and Some Aryl α-Galactosides

Abstract

1. Human kidney α-galactosidase [EC 3.2.1.22], assayed with p-nitrophenyl α-galactoside as a substrate, was separated into two major components, Fr. I and Fr. II, by isoelectric focusing and Cellogel electrophoresis. The isoelectric points were 4.80 and 4.48 for Fr. I and Fr. II, respectively.

2. Thermal inactivation experiments at 55°C revealed that Fr. I was thermolabile and Fr. II thermostable.

3. The Michaelis constant of Fr. I for pNPG at the optimal pH (pH 4.6) was 6.0 × 10⁻³M and that of Fr. II was 13.3×10⁻³M.

4. p-Nitrophenyl α-galactoside hydrolase of Fr. I was inhibited 50% by 400 mM myoinositol. However, Fr. II was insensitive to this inhibitor.

5. Ceramide trihexosidase activity was found in both fractions, and the Michaelis constants were 5.7×10⁻⁴M and 4.7×10⁻⁴M for Fr. I and Fr. II, respectively.

6. The activities of these two fractions toward ceramide trihexoside, 4-methylum-belliferyl α-galactoside and melibiose were parallel with their activities toward p-nitrophenyl α-galactoside.

7. Ceramide trihexosidase activity of the crude enzyme preparation was depressed in high concentrations of protein.