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The role of quinones in electron transport has attracted considerable recent interest (27). Coenzyme Q1,O which is present in plant mitochondria, is closely related to the respiratory chain (6), and the very similar benzoquinone, plastoquinone, appears to be involved in photosynthesis (7); Vitamin K-type naphthoquinones have been implicated in respiratory electron transfer (30), oxidative phosphorylation (2), and photophosphorylation (1). The enzymatic pathways involved in the intracellular oxidation and reduction of these quinones are clearly of interest. Wosilait and Nason (32) showed that extracts of animals, plants, and bacteria contain an active pyridine nucleotide-menadione (vitamin K3) reductase, and since then a number of enzymes which catalyze the reduction of naphthoquinones and benzoquinones or benzoquinones alone have been purified. The pathways of oxidation of the reduced quinones have also been examined, though most of the work has been done with animal systems (8,9,22). This aspect of the problem is of special interest in plants, since, in addition to the usual respiratory chain, they contain a number of enzymes (phenol oxidase, laccase, peroxidase) which can oxidize phenols to their corresponding quinones. In the present study, we have used menadiol and menadione as model compounds and have examined their oxidation and reduction by cell-free fractions isolated from non-photosynthetic tissues. Special attention has been paid to the intracellular localization of these activities and to the components involved in the oxidative pathways.

Oxidation of Menadiol by Fractions Isolated From Non-Photosynthetic Plant Tissues