Regulation of the NAD Malic Enzyme from Crassula

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Abstract
Using size exclusion chromatography, the nicotinamide adenine dinucleotide malic enzyme purified to near homogeneity from leaves of Crassula argentea was found to exist in at least three aggregational states (dimer, tetramer, and octamer). These forms differ in their apparent kinetic characteristics in initial rate assays, but all display similar characteristics at the steady state. The presence of 50 millimolar malate during chromatography causes a shift in favor of the smaller forms with the tetramer predominating. The native enzyme, when diluted 1/1000 and incubated 18 hours in buffer of high ionic strength, changes its steady state kinetic parameters to ones which indicate a low activity and low affinity for malate. When 50 millimolar malate or 50 micromolar coenzyme A are present the loss of activity and increase in Km is reduced. When both malate and coenzyme A are present the effects in minimizing the change in kinetic characteristics are additive.