Purification and characterization of acid phosphatase in aleurone particles of rice grains

The major acid phosphatase (EC 3.1.3.2) associated with aleurone particles of rice grains (Oryza sativa L. Japonica cv. Koshihikari) was purified to homogeneous state by polyacrylamide gel electrophoresis. Its molecular weight was 72,000 when determined by gel filtration and 68,000 when found by polyacrylamide gradient gel electrophoresis in the presence of sodium dodecyl sulfate and β-mercaptoethanol. The purified enzyme had a violet color and an absorption peak at 530 nm. Triton X-100 and lysolecithin stabilized the purified enzyme. The optimum pH for hydrolysis of p-nitrophenyl phosphate was 4.8. The enzyme hydrolyzed all inositol phosphates, several other phosphomonoesters and pyrophosphate. However, α,β-glycerol phosphate, glucose-6-phosphate, adenosine monophosphate and inosine monophosphate were not hydrolyzed. The Km for myo-inositol hexaphosphate was 0.43 mm, which was the lowest among myo-inositol phosphates. The Km value increased as the number of phosphate linkages on myo-inositol decreased. No correlation between the maximum initial velocity (Vmax) and Km was observed. Among the myo-inositol phosphates, the Vmax for myo-inositol triphosphate was the highest. The Km for p-nitrophenyl phosphate was 1.74 mm and that for ATP was 5.26 mm. l-Tartrate, orthophosphate, molybdate and arsenate were competitive inhibitors, and F(-) was a noncompetitive inhibitor. Ag(+), Zn(2+), Hg(2+), Cu(2+) and Fe(2+) were inhibitory and the enzyme was also inactivated by preincubation with EDTA.