Isoenzymes of malate dehydrogenase and their regulation in Euglena gracilis Z

Abstract

The malate dehydrogenase ( l-malate:NAD + oxidoreductase, EC 1.1.1.37) isoenzymes of Euglena gracilis Z occur in two groups, an anodally migrating cluster of three cytoplasmic soluble malate dehydrogenase isoenzymes, which probably occur in the cytosol, and a cathodally migrating group of two particulate isoenzymes which exist in particulate cell components, probably mitochondrial malate dehydrogenase. Comparisons of some physical and kinetic properties are described. Although the molecular and kinetic properties are similar, the soluble malate dehydrogenase is much less stable than mitochondrial malate dehydrogenase. The latter supports the conclusion that the proteins are different. Cells grown heterotrophically in the dark have approximately three times more soluble malate dehydrogenase than cells grown photoautotrophically, whereas there is no significant difference between mitochondrial malate dehydrogenase quantities. This differential regulation between isoenzymes is found when the activities are measured in terms of dry weight of cells, on a unit soluble protein basis, or per cell. Cells transferred abruptly from autotrophic growth conditions to heterotrophic conditions, or vice versa, showed a lag in initiation of soluble malate dehydrogenase modification which correlates with lag in growth. Modulations of the isoenzyme levels in cells changed from one nutritional mode to another were studied under a variety of conditions. In other experiments, no significant alteration of soluble malate dehydrogenase was observed to occur in the absence of growth; removal of any essential parameter for growth ( e.g., CO 2, light, glucose) inhibits soluble malate dehydrogenase changes, i.e., greening of etiolated cells in the absence of CO 2 does not cause a significant reduction of soluble malate dehydrogenase. Analogues of substrates did not induce the soluble malate dehydrogenase, and the effects of inhibitors upon the enzyme changes was studied with paradoxical results.