Partial purification and characterization of soluble acid invertases from rice (Oryza sativa) leaves

There are three soluble acid invertases in the leaves of rice. They could be separated by DEAE1-Sephacel chromatography, and named IT1, IT2 and IT3. The molecular weights of IT1, IT2 and IT3, determined by HPLC gel filtration chromatography, were 220 kD, 59 kD and 67 kD, respectively. Their pI values were 6.2, 5.2 and 4.9, respectively. The pH optima of IT1, IT2 and IT3 were 3.5, 6.0 and 5.5 respectively. All of them could hydrolyze sucrose and raffinose but not maltose, therefore they are all beta-fructofuranosidases. The apparent Km values for sucrose and raffinose of IT3 were 7.0 and 14.5 mM, respectively. Exogenous protein BSA could activate IT3. IT3 was inhibited by metal ions--Pb2+, Zn2+, Cu2+, Hg2+ and Ag+. The sulfhydryl group inhibitor, PCMB, and serine protease inhibitor, PMSF, had no effects on IT3 activity. It might indicate that cysteine and serine did not participate directly the active site catalytic reaction of IT3.