Influence of Oxygen and Temperature on the Dark Inactivation of Pyruvate, Orthophosphate Dikinase and NADP-Malate Dehydrogenase in Maize.

The influence of oxygen and temperature on the inactivation of pyruvate, Pi dikinase and NADP-malate dehydrogenase was studied in Zea mays. O(2) was required for inactivation of both pyruvate, Pi dikinase and NADP-malate dehydrogenase in the dark in vivo. The rate of inactivation under 2% O(2) was only slightly lower than that at 21% O(2). The in vitro inactivation of pyruvate, Pi dikinase, while dependent on adenine nucleotides (ADP + ATP), did not require O(2).The postillumination inactivation of pyruvate, Pi dikinase in leaves was strongly dependent on temperature. As temperature was decreased in the dark, there was a lag period of increasing length (e.g. at 17 degrees C there was a lag of about 25 minutes) before inactivation proceeded. Following the lag period, the rate of inactivation decreased with decreasing temperature. The half-time for dark inactivation was about 7 minutes at 32 degrees C and 45 minutes at 17 degrees C. The inactivation of pyruvate, Pi dikinase in vitro following extraction from illuminated leaves was also strongly dependent on temperature, but occurred without a lag period. In contrast, NADP-malate dehydrogenase was rapidly inactivated in leaves (half-time of approximately 3 minutes) during the postillumination period without a lag, and there was little effect of temperature between 10 and 32 degrees C. The results are discussed in relation to known differences in the mechanism of activation/inactivation of the two enzymes.