Regulation of steady state pyruvate dehydrogenase complex activity in plant mitochondria : reactivation constraints

The requirements for reactivation (dephosphorylation) of the pea (Pisum sativum L.) leaf mitochondrial pyruvate dehydrogenase complex (PDC) were studied in terms of magnesium and ATP effects with intact and permeabilized mitochondria. The requirement for high concentrations of magnesium for reactivation previously reported with partially purified PDC is shown to affect inactivation rather than reactivation. The observed rate of inactivation catalyzed by pyruvate dehydrogenase (PDH) kinase is always greater than the reactivation rate catalyzed by PDH-P phosphatase. Thus, reactivation would only occur if ATP becomes limiting. However, pyruvate which is a potent inhibitor of inactivation in the presence of thiamine pyrophosphate, results in increased PDC activity. Analysis of the dynamics of the phosphorylation-dephosphorylation cycle indicated that the covalent modification was under steady state control. The steady state activity of PDC was increased by addition of pyruvate. PDH kinase activity increased threefold during storage of mitochondria suggesting that there may be an unknown level of regulation exerted on the enzyme complex.  

Light as a signal influencing the phosphorylation status of plant proteins

The phosphorylation-status of a number of plant enzymes has been shown to be altered in response to light. Phosphoenolpyruvate carboxylase is phosphorylated (more active) in C₄ plants in the light but CAM phosphoenolpyruvate carboxylase is phosphorylated (more active) in the dark. C₄ plant pyruvate, Pi dikinase is dephosphorylated (activated) in the light and sucrose phosphate synthase is less phosphorylated (more active) in the light. The mitochondrial pyruvate dehydrogenase is inactivated (phosphorylated) in the light. The reversal of these events occurs in the dark or when photosynthesis is inhibited. Phytochrome and blue light receptors also alter the phosphorylation-status of proteins. The evidence is rapidly increasing in support of signal transduction networks in plants that involve light reception.