Properties of Leaf NAD-Malic Enzyme from the Inducible Crassulacean Acid Metabolism SpeciesMesembryanthemum crystallinum

摘要

NAD-malic enzyme (NAD-ME) functions to decarboxylate malate in the light in leaves of certain species displaying Crassulacean acid metabolism (CAM). The properties of NAD-ME in desalted extracts from the inducible CAM species,Mesembryanthemum crystallinumwere examined. The shapes of the malate saturation curve and the activity versus pH curve at 10 mM malate were dependent on the presence of the activator CoA. The malate saturation curve was sigmoidal in the absence of an activator and hyperbolic in the presence of CoA. The pH optimum with 10mmmalate and Mn2+as cofactor was as low as 6.5 without an activator, and increased to 7.2 in the presence of CoA. Fumarate activation was synergistic with CoA above pH 7.2. The enzyme displayed hysteretic behavior under suboptimal assay conditions.Rapid extraction and desalting of the enzyme (<1.5 mim) followed immediately by assay did not reveal any difference in the properties of the enzyme on a day/night basis. It is proposed that diurnal regulation of the enzyme in vivo is mediated by pH and malate level without a change in the oligomeric form of the enzyme. The molecular weight of the enzyme was approximately 350,000 at pH 6.5 or 7.8. The enzyme obtained fromM. crystallinumin the C3mode was very similar to the CAM enzyme except that it displayed a lower Vmax.