beta-Glucosidases in the rice weevil, Sitophiulus oryzae: purification, properties, and activity levels in wheat- and legume-feeding strains

beta-Glucosidase activity in the rice weevil, Sitophilus oryzae (L.), measured with the substrate 4-methylumbelliferyl-beta-D-glucoside (MU beta Glu), is present in the soluble fraction of gut tract homogenates (105,000 g supernatant) with 13% found in foregut and 82% found in midgut regions. About 16% of beta-glucosidase activity in whole body and gut tract homogenates was lost after storage for 16 weeks at -70 degrees C. However, stability of the enzyme was generally independent of buffer pH from pH 5.5 to 8.5. A minor and a major beta-glucosidase fraction, peaks I and II, respectively, were partially-purified from S. oryzae by ammonium sulfate precipitation and chromatography on DEAE-Sepharose CL6B. Relative substrate specificities (Vmax.Km-1) of peak I (R(m) 0.51) were helicin > amygdalin > NP beta Glu > cellobiose >> salicin. Relative substrate specificities of II (R(m) 0.59 were cellobiose > helicin > p-nitrophenyl-beta-D-glucopyranoside (NP beta Glu) > amygdalin >> salicin. Arbutin, phloridzin, and methyl-beta-glucopyranoside were poor substrates for both fractions. Molecular masses of peaks I and II were estimated to be 140 and 170 kDa by gel filtration, respectively. There were no qualitative differences between beta-glucosidase activity in the Savannah strain of S. oryzae and two rice weevil strains, Burma and Trinidad, adapted to yellow split pea, as determined by electrophoretic zymogram analysis with the substrate MU beta Glu. In addition, activities in the Savannah strain against cellobiose, NP beta Glu, helicin, amygdalin, and salicin were intermediate between those of Burma and Trinidad. When weevils from the Burma and Trinidad strains were transferred to wheat for 3 weeks, beta-glucosidase activity based on gut equivalents was reduced. Nevertheless, based on results with these tested substrates, there is no obvious relationship between beta-glucosidase activity in adults of these rice weevil strains and their adaptation to feeding and breeding on legumes.