Isoenzyme Spectrum, Localization and Some Physicochemical Properties of NAD-Malate Dehydrogenase in Amaranth Leaves Under Drought

Malate dehydrogenase (MDH) plays a crucial role in energy and cell metabolism. Activity, subcellular distribution, isoenzyme spectrum and kinetic properties of cytosolic (CMDH) and mitochondrial malatdehydrogenases (mMDH) have been studied in the phases of pre-anthesis, anthesis and grain ripening. Subcellular localization, isoenzyme spectrum and kinetic properties of NAD-malate dehydrogenase (1-malate-NAD-oxidoreductase, NAD-MDH, EC 1.1.1.37) were studied in subcellular fractions (SCFs) of assimilating tissues of Amaranthus cruentus L. leaves during pre-anthesis, anthesis and grain ripening phases. NAD-MDH was found to have a wide isoenzyme spectrum in amaranth leaves, which changed depending on the number of isoenzymes, type of tissues and SCFs. Main part of the enzyme general activity was localized in cytosolic and mitochondrial fractions of both tissues. V (OAA) value of the reaction catalyzed by cNAD-MDH in mesophyll cells (MC) were 2-times higher compared with Vmas (OAA) of the reaction catalyzed by mNAD-MDH. Whereas, in bundle sheath cells (BSC) V (OAA) for mNAD-MDH was 3-4-times higher than the same parameter of the enzyme in MC. The enzyme isoforms had a wide range of pH optimum and were tolerant to temperature. Depending on SCFs the enzyme had higher sensitivity to the substrate oxloacetate (OAA) and lower sensitivity to malate. The kinetics of the enzyme follows Michaelis-Menten plot and this enzyme is not allosteric. The enzyme activity is strongly regulated by substrates, mono and bivalent ions. Inhibitory effect of ATP to the all isoforms even more increased under drought