Metabolism of micronutrient elements in higher plants. II. Effect of copper deficiency on the isocitric enzyme in tomato leaves.

The r81e of copper as a component of such enzymes as ascorbic acid oxidase and polyphenol oxidase has been well established (1, 2). When plant tissues were made copper-deficient, the activities of these particular oxidases were significantly diminished, while other enzymes were unchanged or increased in concentration, resulting in an over-all change in enzyme pattern characteristic of the metal deficiency (3). However, when an increase in the activity of an enzyme occurred, it was not characteristic of a copper deficiency alone, since certain other metal deficiencies produced a similar response. These increases were attributed to metabolic changes in the synthesis of the protein moieties of the enzymes concerned. Thus far the only causal relationship between an increased enzyme concentration and a specific metal deficiency has been noted for a DPNase* in zinc-deficient Neurospora (4). However, the experiments reported in this paper demonstrate another such specific relationship: that cell-free extracts of copper-deficient tomato leaves show an approximately 15-fold increase in isocitric dehydrogenase activity as compared to normal leaves or those deficient in other metals. A number of workers have formulated the enzymatic conversion of isocitrate to a-ketoglutarate in heart muscle (5), liver (6), higher plants (7), and microorganisms (8) by the following reactions.