Effect of Manganese and Certain Other Metal Cations on Isocitric Dehydrogenase and Malic Enzyme Activities in Phaseolus vulgaris.

Abstract

Tissue extracts from Mn-deficient or normal plants catalyzed very little dehydrogenation of isocitrate or malate unless Mn++ was added to the assay reaction mixture. Extracts of Mn-toxic plants, however, showed considerable dehydro-genase activities without exogenous Mn++. When assayed with Mn++ in the reaction mixture, Mn-deficient tissues were found to contain as much isocitric dehydrogenase and "malic" enzyme as the tissues of normal plants. Leaf and root extracts of Mn-toxic plants showed a 2- to 3-fold increased isocitric dehydro-genase activity. This increased activity was decreased to near-normal by adding high concentrations of Fe or Al to culture solutions of Mn-toxic plants or was nearly prevented if large amounts of Fe or Al were added at the same time as the toxic concentration of Mn was applied. Metal ions affected the "malic" enzyme activity of leaves in the same way as they affected isocitric dehydrogenase activities of leaves and roots; however, an almost reverse relationship of the metals on "malic" enzyme activity of root extracts was found. The effect of some other metal toxicities and deficiencies on the activities of these enzymes is reported. With respect to Mn++, isocitric dehydro-genase and "malic" enzyme activities were noncompetitively inhibited by low concentrations of Fe+++ and A1+++ and by higher amounts of Cu++. The inhibition was mainly of a noncompetitive nature but showed some tendency to influence the dissociation of the enzyme-Mn++ complex. The relationship of metal ion culture level to isocitric dehydrogenase-and "malic" enzyme constitution is discussed in connection with the Fe-Mn inter-action on growth.