Some molecular properties of rat-liver lysosomal β-glucuronidase isoenzymes

The isoenzymes of rat-liver lysosomal β-glucuronidase (β-D-glucuronide glucuronosohydrolase (EC 3.2.1.31)) were inactivated at different rates at 0 °C in 3 M guanidinium chloride solutions adjusted to pH 5.0. In 4 M urea buffered by 0.01 M glycylglycine, pH 7.0, isoenzymes I, III, and V were reversibly inhibited 80%. Sodium dodecyl sulfate (SDS), 0.1% in 0.01 M phosphate buffer, pH 7.0, irreversibly inhibited at 37 °C all five isoenzymes. Sedimentation analysis showed that loss of catalytic activity in these denaturing media is accompanied by dissociation into slower sedimenting subunits. SDS gel electrophoresis revealed that the isoenzymes are apparently tetramers made up of different proportions of subunits α, β, and γ having apparent molecular weights of 62 900,60 200, and 58 700, respectively. The three subunits appear to be glycoproteins.