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Articles

Metabolism of trans-Aconitic Acid in Maize ¹

II. Regulatory Properties of Two Compartmented Forms of Citrate Dehydrase

Department of Plant Science, University of Delaware, Newark, Delaware 19711

Kinetics of two molecular forms of K-dependent citrate dehydrase in maize (Zea mays L.) are reported. The isozymes, designated CD I and CD II, were found to be compartmented in mitochondria and cytosol, respectively.

CD I exhibited hyperbolic kinetics with respect to both citrate and potassium with K_m 2.3 and 12 millimolar, respectively. Maximum velocity was 0.38 micromole of trans-aconitic acid per minute per milligram protein. The pH optimum was 7.2. trans-aconitic synthesis by CD I is regulated by both citrate concentration and pH.

CD II exhibited hyperbolic kinetics with respect to citrate (K_m 0.6 millimolar) but sigmoidal kinetics with respect to potassium. trans-aconitic acid synthesis by CD II is regulated by potassium. This may account for the positive correlation between leaf potassium and trans-aconitic acid in certain grasses (Clark 1968 Crop Sci 8: 165).

¹ Published with the approval of the Director of the Delaware Agricultural Experiment Station as Miscellaneous Paper No. 973. Contribution No. 117 of the Department of Plant Science.