NADP-malic enzyme from maize leaf: Regulatory properties

The regulatory properties of purified maize leaf NADP-malic enzyme (EC 1.1.1.40) were studied at three different pHs and the following results were obtained. (a) At pH 7.5 enzyme activity reaches a maximum at 0.4–0.8 mm malate depending on the Mg2+ concentration, and higher levels of malate result in marked substrate inhibition; with increasing pH the degree of substrate inhibition is reduced to where at pH 8.4 little or no inhibition is observed. (b) The inhibitory effect of malate is more pronounced at 1 mm Mg2+ than at 5–10 mm Mg2+ in the pH range of 7.5 to 8.4; a plot of enzyme activity vs Mg2+ concentration at 3 mm malate follows Michaelis-Menten kinetics at both pH 7.5 and 8.4; the apparent affinity of the enzyme for Mg2+ at pH 8.4 was threefold greater than that at pH 7.5. (c) The activity of NADP-malic enzyme decreases as the ratio of NADPHNADP increases, and this effect is enhanced at lower pH. (d) Various α-keto acids including glyoxylate, oxaloacetate, and α-ketoglutarate inhibit NADP-malic enzyme activity, whereas HCO3−, pyruvate, and other organic acids, sugar phosphates, and amino acids have little or no effect on the activity of the enzyme. Based on these experimental findings, the regulatory properties of maize leaf NADP-malic enzyme are discussed with respect to its key role in net CO2 fixation in maize bundle sheath chloroplasts during C4 photosynthesis.