Function and regulation of NADP-specific malate dehydrogenase in Euglena gracilis Z

Abstract

NADP-specific malate dehydrogenase (decarboxylating) ( l-malate:NADP oxidoreductase (decarboxylating), EC 1.1.1.40) was found to predominate in Euglena cytosol. A single peak of enzyme activity elutes from a DEAE-cellulose column and from a Sephadex G-200 column. The specific activity of the enzyme is low in cells grown autotrophically, i.e. in the light, with CO 2 as the sole carbon source, and varies little throughout the growth cycle. In cells grown heterotrophically, i.e. in the dark, with glucose as the sole carbon source, the specific activity of the dehydrogenase increases linearly throughout the growth cycle. The specific activity in cells grown heterotrophically is always greater than in autotrophically grown cells; the maximum difference observed is of the order of 55-fold. Modulations of the dehydrogenase were studied under a variety of nutritional conditions. Exposure of heterotrophically grown cells to the light always causes a reduction of NADP-specific malate dehydrogenase, whatever the conditions of CO 2 and glucose. This reduction is usually exponential and can occur in the absence of growth. The specific activity of the dehydrogenase in autotrophically grown cells is not affected by removal of CO 2, or light, or both. Nor is it affected by addition of glucose, or addition of glucose and removal of CO 2, provided that the cultures are left in the light. Gassing heterotrophic cells with CO 2 in the dark has no effect upon the specific activity of the dehydrogenase. Inhibition of growth and chlorophyll biosynthesis by chloramphenicol or dl-ethionine does not inhibit this light-induced reduction of the specific activity of malate dehydrogenase. The possibility of a shunt bypassing the pyruvate kinase (EC 2.7.1.40) step of glycolysis, and involving phosphoenolpyruvate carboxylase, soluble malate dehydrogenase and NADP-specific malate dehydrogenase is discussed in terms of a possible function in regulation of the ratio between NADH and NADPH.