De novo synthesis and developmental control of the multiple gene-controlled malate dehydrogenase isozymes in maize scutella.

In maize, both soluble and mitochondrial malate dehydrogenases (L-malate:NAD oxidoreductase, EC 1.1.1.37)exist in multiple molecular forms (isozymes). Biochemical studies and genetic analysis showed that the maize malate dehydrogenase isozymes are not interconvertable conformational forms of a single gene product. Instead, they are determined by multiple genes which may reside on different chromosomes. Therefore, developmental expression of the various malate dehydrogenase isoqymes was studied. During early development of the sporophyte (dry kernel to 10 days of germination), the total malate dehydrogenase activity in scutella increases through the first 5 days, peaks about the 6th day and decreases gradually thereafter. Both soluble and mitochondrial malate dehydrogenase isozymes exhibit similar activity profiles; however, the total mitochondrial malate dehydro genase activity is only 60% of that in the cytosol. Density labeling experiments and atempts to detect the possible existence of "inactive malate dehydrogenase precursors" were performed. The results show that both soluble and mitochondrial malate dehydrogenase are synthesized in the scutella during germination and early growth of young maize seedlings. Accumulation of the maize malate dehydrogenase isozymes is probably controlled by synthesis as well as degradation instead of activation and inactivation of enzyme moieties. Correlation between the developmental pattern of the scutellar malate dehydrogenase isozymes and the physiological conditions of the young maize seedlings is discussed.