Allosteric regulation of the NAD malic enzyme from cauliflower: activation by sulfate

Activation of the NAD malic enzyme by sulfate has been found to be due only to free, uncomplexed SO42-; the complex of sulfate with divalent cations has no measurable effect on the enzyme. Activation by SO42- is shown to result from a decrease in the Km for malate2-. Thus, activation is observed only at less than saturating levels of this substrate. The interactions of NAD+ and Mg2+ with the enzyme are not affected by SO42-. Response of the activity of the enzyme to SO42- is biphasic in that the activation seen at low SO42- concentrations is overcome as the level of the effector is increased so that at very high SO42- concentrations, activation disappears. This deactivation process is not simply a reversal of the activation mechanism; instead, it involves a decrease in the intrinsic Vmax of the reaction. The response to SO42- is also affected by the presence of other anionic effectors of the malic enzyme. Fumarate2- and phosphate are shown to directly affect the activation process by increasing the affinity of the enzyme for SO42-. While Cl- does not greatly affect the extent of stimulation, it does inhibit the enzyme without reducing the activated rate so that the apparent percentage activation over the control is very large, due to the lowered control rate. In contrast to the sensitivity of the malic enzyme reaction to pH, activation by SO42- appears to be independent of H+ concentration. The possibility that sulfate is a physiological effector of this and other plant mitochondrial enzymes is discussed.