Dioxygenase and hydroperoxidase activities of rat brain cytosolic lipoxygenase.

Dioxygenase and hydroperoxidase activities of dialyzed rat brain cytosolic lipoxygenase were studied using linoleic acid as the substrate. The formation and utilization of linoleic acid hydroperoxide was monitored spectrophotometrically at 234 nm. Dioxygenase activity was dependent upon the concentration of linoleic acid and enzyme. The optimum assay conditions necessary for maximal dioxygenase activity included the 0.5 mM linoleic acid, cytosol (100 micrograms of protein) as the enzyme source and pH 7.4. All the classical inhibitors of lipoxygenase, nordihydroguaiaretic acid, phenidone and 5,8,11-eicosatriynoic acid significantly inhibited dioxygenase activity in a dose dependent manner. A significant hydroperoxidase activity was observed towards xenobiotic substrates viz. benzidine, guaiacol, tetramethylphenylenediamine, thiobenzamide in the presence of linoleic acid. NDGA at 1 microM concentration inhibited benzidine oxidation (2.45 nmol product formed/min/mg protein) by 75%, while Phenidone at 0.125 microM and ETI at 20 microM inhibited benzidine oxidation by 30% and 25%, respectively.