Tartaric Acid Dehydrogenase Activity in Higher Plants

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Abstract
A dehydrogenase activity which oxidizes (-) -or meso- tartarate to either the enol or keto form of dihydroxyfumar-ate has been demonstrated in pea (Pisum sativum), bean (Phaseolus vulgaris), and wheat germ (S-50) extracts. This activity has been measured spectrophotometrically with substrate amounts of DPN+ or colorimetrically by the Prussian blue analysis of the K4Fe(CN)g formed in a system coupled with the oxidation of K3Fe(CN)g. Under the conditions used, neither Mg++ nor ethylenediaminetetraacetic acid affects the rate of this activity. The reduction of dihydroxyfumarate to tartrate in the presence of DPNH can be demonstrated spectrophotometrically under anaerobic conditions in certain preparations. Identification of the products of the dehydrogenase activity is hampered by at least three side reactions; the decarboxylation of dihydroxyfumarate to glycolaldehyde; the autoxidation of dihydroxyfumarate to diketosuc-cinate followed by a decarboxylation to hydroxymalonate (tartronate); the possible dehydration of tartrate to oxalacetate. No evidence of significant amounts of glyoxylate or hydroxypyruvate was obtained. The dehydrogenase activity is found in seeds, roots, and shoots of peas. Although the activity is associated mainly with the supernatant fraction, a definite activity can be demonstrated in a partially purified particulate fraction sedimented between 500 to 25,000 x g. The possibility that this activity is merely a secondary one of the similarly distributed malic dehydrogenase has not yet been eliminated. Like hydroxymalonate, tartrate is a competitive inhibitor of malic dehydrogenase activity. The possible key role of dihydroxyfumarate in plant metabolism is discussed.