Studies on rice bran lipoxygenase:purification, characterization and kinetic mode of its action

Abstract

Lipoxygenase (EC 1. 13. 11. 12) which catalyzes hydroperoxidation of polyunsaturated fatty acids by direct insertion of molecular oxygen was purified from the water soluble extract of rice bran by conventional methods through ammonium sulfate fractionation, gel filtration using Sephadex G-150, and ion exchange column chromatography on TEAE-cellulose. The enzyme was purified 85 fold with the yield of 26%. It was found that the enzyme showed its optimum pH and temperature at pH 6.8-pH 7.0 and 30℃, respectively. The enzyme was reasonably stable at room temperature and at neutral pH. Molecular weight determined by gel filtration was 100,000. The enzyme was highly specific to free fatty acid containing cis, cis-1,4-pentadiene in its structure and the reaction product was identified to posess trans, cisconjugated butadiene system and hydroperoxy group. The enzyme activity was reduced markedly in the presence of chelating agents, commercial antioxidants, and copper ion. Initial velocity patterns obtained by varying both concentrations of linoleate and oxygen were non-parallel type, which suggested that the enzymic hydroperoxidation proceeded through the formation of intermediate ternary complex between the enzyme and two ligand substrates. Native enzyme showed EPR signal at g 3.0 and 4.3 due to the existence of ferric ion in the enzyme molecule and the trivalent iron was converted to ferrous state as the enzyme reaction proceeded. Involvement of imidazole group during the catalytic action of the enzyme was identified by photooxidation along with kinetic pH-rate profile. Based on the above results mode of rice bran lipoxygenase action was proposed.