Aggregation properties of beta-galactosidase of human urine and degradation of its natural substrates by a purified preparation of the enzyme.

ABSTRACT

Acid beta-D-galactosidase (beta-D-galactoside galactohydrolase, EC 3.2.1.23) was purified to near homogeneity from normal human urine by two affinity chromatography steps. On polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate the major protein band had an apparent molecular weight of 59000, thus being 5000 daltons smaller than the protein purified from human liver. Upon gel filtration on Sephadex G-150 column the purified enzyme had an apparent molecular weight of 70000 of pH 7.0. At pH 4.0 partial aggregation to a dimer of an apparent molecular weight of 150000 was found. Addition of 0.1 M galactose caused at pH 3.5, but not at pH 4.0 and 7.0, an increased formation of multimeric beta-galactosidase which eluted with the void volume of the column. Crude beta-galactosidase from human urine showed a higher aggregation tendency than the purified enzyme. None of the conditions produced an enzyme species of an apparent molecular weight of less than 40000. pH-activity profiles were measured against p-nitrophenyl-beta-D-galactoside, 3H-labelled GM1-ganglioside, [3H]keratan sulfate and the pentasaccharide O-beta-(1 leads to 4)-[6-3H]galactopyranosyl-O-beta-(1 leads to 2)-2-deoxy-2-acetamidoglycopyranosyl-O-alpha-(1 leads to 6)-mannopyranosyl-O-beta-(1 leads to 4)-mannopyranosyl-2-deoxy-2-acetamidoglucopyranoside. While p-nitrophenyl-beta-D-galactopyranoside and GM1-ganglioside were optimally hydrolyzed at pH 4.0, keratan sulfate and the pentasaccharide were optimally degraded at pH 4.3 and pH 5.0, respectively. With the chromogenic substrate and with GM1-ganglioside Km values of 0.33 mM were calculated. At pH 3.5 the hydrolysis of the synthetic substrate did not follow Michaelis-Menten kinetics. Two enzyme species appeared with Km values of 0.006 mM and 3.2 mM, respectively. The affinity of beta-galactosidase for [3H]keratan sulfate and the 3H-labelled pentasaccharide was at least one order of magnitude lower than for the amphiphilic substrates. Keratan sulfate and GM1-ganglioside did not act as competitive inhibitors of p-nitrophenyl-beta-galactosidase at the concentration tested. These findings could be explained by the existence of different binding sites for the substrates used.