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 reducta
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