Regulation of pea mitochondrial pyruvate dehydrogenase complex : does photorespiratory ammonium influence mitochondrial carbon metabolism?

Inactivation of the pyruvate dehydrogenase complex catalyzed by pyruvate dehydrogenase kinase was studied using intact mitochondria purified from green leaf tissue of pea (Pisum sativum L.) and dialyzed mitochondrial extracts. Thiamine pyrophosphate was inhibitory in dialyzed extracts but not in intact mitochondria, except in the presence of high concentrations of Na(+). NH(4) (+), at concentrations as low as 20 micromolar, markedly stimulated inactivation in dialyzed extracts. K(+) in the range 1 to 10 millimolar also enhanced inactivation. In contrast, Na(+) was without affect at lower concentrations but was inhibitory at 10 to 100 millimolar levels. The effect of NH(4) (+) is discussed in relation to a possible regulatory interaction between photorespiratory NH(4) (+) production and the entry of carbon into the tricarboxylic acid cycle by way of the pyruvate dehydrogenase complex.