PHYSIOLOGICAL AND BIOCHEMICAL STUDIES IN PLANT METABOLISM: II. RESPIRATORY ENZYMES IN WHEAT

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Abstract
The kinetics of the malic dehydrogenase in cell-free extracts of wheat seedlings have been studied by the Warburg technique, using methylene blue. The enzyme behaves analogously to the animal enzyme in that it is coenzyme I-linked and its oxidation product, oxalacetic acid, is inhibitory to the reaction. Oxalacetic acid is removed by either a ketone fixative or l(+) glutamic acid, the latter by being involved in a transamination. Cell-free extracts also contain peroxidase and ascorbic acid oxidase, but no phenolases. In embryonic tissue the succinoxidase system, including cytochrome oxidase, is present. Using l(+) glutamic acid to remove oxalacetic acid, ascorbic acid can replace methylene blue and function as a respiratory carrier mobilizing the hydrogen from dihydrocoenzyme I indirectly via a pigment of unknown chemical nature. A proposed carrier system for the wheat plant includes coenzyme I, flavin, pigment, and ascorbic acid. In vitro, the entry of oxygen to the system is catalyzed by ascorbic acid oxidase supported by a thermostable copper-catalyzed reaction. Alcohol, hexosediphosphate, and fumarate can donate hydrogen to this carrier system.