ENZYMIC OXIDATION OF SIMPLE DIPHENOLS AND FLAVONOIDS BY ORANGE JUICE EXTRACTS

SUMMARY: Extracts of orange juice vesicles oxidized p-hydroquinone (HQ) and o-dihydroxyphenylalanine (DOPA) with O2. Most of the oxidase activity was associated with a particulate fraction that sedimented at 100,000 × g. Sonic disruption of the particulates followed by chromatography on DEAE-cellulose increased specific activities with both substrates. The partially purified enzyme oxidized numerous naturally occurring o-diphenols and o-methoxyphenols. Acidic media below pH 4 and heating above 70°C destroyed most of the oxidase activity. KCN, diethyl-dithiocarbamate and 8-hydroxyquinoline but not EDTA inhibited the oxidase. The partially purified enzyme reduced benzoquinone and oxidized the reduced form of nicotinamide-dinucleotide (NADH). Ubiquinone, a benzoquinone derivative, but not menadione or vitamin K1, naphthoquinone derivatives, could replace benzoquinone in the oxidation of NADH. Ubiquinone, benzoquinone and similar p-quinones may function in the orange as the oxidation-reduction couple between NADH-quinone reductase and diphenol oxidase.