Influence of Environmental Factors on the Light Activation of Pyruvate, PI Dikinase and NADP-Malate Dehydrogenase in Maize

The interaction among several environmental factors on the degree of light activation of NADP- malate dehydrogenase (MDH, EC 1.1.1.82) and pyruvate,PI dikinase (EC 2.7.9.1) in the C4 plant maize was determined. At 25°C under limiting light (50 �E m-2 s-1), there was a several-fold stimulation in the degree of activation of both enzymes when CO2 was removed from the atmosphere. Under high light (1000 �E m-2 s-1), CO2 did not influence the degree of activation of the enzymes. At 25°C under limiting light in the absence of CO2, the degree of activation of NADP-MDH increased as O2 was reduced below 21% with maximum activation occurring under anaerobic conditions. O2 had little or no effect on the degree of activation of NADP-MDH under high light. In contrast, with pyruvate,PI dikinase at 25°C and in the absence of CO2, anaerobic conditions strongly inhibited the degree of activation of the enzyme under low or high light, while under aerobic conditions the enzyme was activated to the same extent under 2 or 21% O2. The inhibition of activation of the dikinase under anaerobic conditions was partially relieved by addition of atmospheric levels (0.03%) of CO2. In contrast to the results at 25°C, the presence or absence of CO2 or O2 at 10°C under low light had little or no effect on the activation of the two enzymes. The results are evaluated in relation to previous in vitro studies which suggest activation of NADP- MDH by reduction of disulfide bonds on the enzyme and activation of pyruvate,PI dikinase by changes in adenine nucleotides.