Partial purification and characterization of invertase isozymes from rice grains (Oryza sativa)

Four invertase isozymes have been isolated from the milky stage rice grains. According to the pH optima, they are classified as one alkaline (IT7) and three acid invertases. The acid invertases are further divided into two soluble forms (IT4 and IT5) and one cell wall-bound (ITb) form which was solubilized in 1 M NaCl. The pH optima of ITb, IT4, IT5 and IT7 are 4.5, 3.5-4.0, 5.0 and 7.0, and the molecular masses are 42, 60, 64 and 260 kDa, respectively. Both IT4 and IT5 were bound to Con A-Sepharose suggesting that these enzymes are glycoprotein. The Km of ITb, IT4, IT5 and IT7 for sucrose are 4.3, 0.9, 12.1 and 70.1 mM, respectively. IT4 and IT5 have a higher Km for raffinose, and the maximum activities are 64% and 27% of that using sucrose as the substrate. IT7 did not hydrolyze raffinose at all. These invertases also exhibit distinct isoelectric points (pI) and different susceptibility to various inhibitors.