Purification and properties of a cysteine proteinase from germinating rice seeds.

A proteinase found in rice seeds during germination was purified by (NH4)2SO4 fractionadon, CM-Sephadex chromatography, Sephadex G-100 gel filtration, Phenyl-Sepharose CL-4B chroraatography, and chromatofocusing. The purified enzyme had a molecular weight of about 23, 000 and an isoelectric point at pH 5.15. Its optimum pH for hydrolysis lay at 5.5 and at 5.75 when measured with α-N-benzoyl-DL-arginine-2-naphthylamide and casein, respectively. The proteinase was activated by thiol compounds and it was inhibited by common thiol-blocking reagents, particularly by leupeptin, antipain, and E-64. It was also inhibited, in a non-competitive manner, by a cysteine proteinase inhibitor purified from rice seeds. These results suggests that this enzyme belongs to the cysteine proteinase class (EC 3.4.22).