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

Abstract

The influence of O2 and temperature on the inactivation of pyruvate, Pi dikinase and NADP-malate dehydrogenase in the dark in vivo. The rate of inactivation under 2% O2 was only slightly lower than that at 21% O2. The in vitro inactivation of pyruvate, Pi dikinase, while dependent on adenine nucleotides (ADP and ATP), did not require O2. 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.degree. C there was a lag of .apprx. 25 min.) before inactivation proceeded. Following the lag period, the rate of inactivation decreased with decreasing temperature. The half-time for dark inactivation was .apprx. 7 min at 32.degree. C and 45 min at 17.degree. 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. NADP-malate dehydrogenase was rapidly inactivated in leaves (half-time of .apprx. 3 min) during the postillumination period without a lag and there was little effect of temperature between 10.degree. and 32.degree. C. Differences in the mechanism of activation/inactivation of the 2 enzymes were discussed.