104.
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°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°C and 45 minutes at 17°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°C. The results are discussed in relation to known differences in the mechanism of activation/inactivation of the two enzymes.
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