Tartaric Acid Dehydrogenase Activity in Higher Plants1,2

Although the tartrates have been considered as possible metabolites in higher plants since the initial work of Pasteur in 1848, surprisingly little is known of their actual physiological significance. Vickery and Palmer (35) have briefly reviewed the history of (+)-tartrate in plants, including the problems of identification and of nomenclature. They conclude that (+)-tartrate is an uncommon plant acid produced from glucose by a few plants like grapes. In their own experimental work with tobacco leaves, these authors report that the (+)-isomer is metabolically inert, and that it has no effect on the production of the enzymatically active L-malic acid. No reference is made, however, to either the mesoor the (-)-forms as possible metabolites, the latter isomer having the configuration of the enzymatically active L-malic acid. The meso-form could have been overlooked in plants because it has solubility characteristics which differ from those of the other tartrates. Bacteria and molds, on the other hand, can oxidize both the (-)and (+)-isomers of tartaric acid, the (+)-isomer being the most active (34). Recently, bacterial extracts have been reported capable of converting either (-)or (+)-tartrate to oxalacetate (17, 20), and tartrate is postulated as an intermediate in the production of oxalate from oxalacetate in Aspergillus niger (2). Furthermore, the (-)and meso-isomers of tartrate are active metabolites in extracts of animal tissues. Scholefield (28) investigated a DPN-linked dehydrogenase in pigeon liver which oxidizes meso-tartrate, while Kun et al (15, 16) reported a mitochondrial dehydrogenase in rat and beef tissue which is capable of oxidizing both the (-)and meso-forms, but not the (+) -isomer. The presumed oxidation product, dihydroxyfumarate (DHF) has been reported in higher plants (7, 18, 27), but the instability of this compound would make its identification very difficult. Furthermore, the presence of DHF could easily be confused with ascorbic acid because of similar chemical properties (7).