Purification and partial characterization of β-glucosidase from plasmodial membrane and culture medium of Physarum polycephalum

β-Glucosidase activity was detected in the plasma-membrane fraction of plasmodia of Physarum polycephalum. The enzyme activity was also found in the extracellular (medium) fraction after maximal growth had been attained, though the activity in this fraction was negligible during the exponential growth phase. The specific activities in both fractions were increased appreciably when the plasmodia were cultured in a glucose-free medium. The β-glucosidase was purified from the culture fluid by gel-filtration, ion-exchange and hydrophobic chromatographies. The purified enzyme appeared homogeneous on polyacrylamide-gel electrophoresis, and the molecular mass was estimated to be about 65 kDa. The enzyme showed highest activity at pH 4.5 and at 55 °C. The β-Glucosidase from the membrane fraction showed very similar properties to that from the culture medium, suggesting that the extracellular enzyme was derived from the membrane. The enzyme was active on laminarin and lichenan as well as on p-nitrophenyl-β-D-glucoside; it was strongly inhibited by D-glucono-1,5-lactone, N-bromosuccinimide and Hg²⁺.