Pyruvate dehydrogenase complex from higher plant mitochondria and proplastids: regulation [Peas, castor-beans]

The activity of the pyruvate dehydrogenase complex from pea (Pisum sativum L.) mitochondria is inhibited when MgATP is added to the reaction mixture; 50% inhibition occurs at 4 mM ATP. The inhibition does not increase with time and is higher in the more highly purified preparations. Crude preparations of the complex show a time-dependent inactivation when incubated with 7.5 mM MgATP alone but this is not found with the more highly purified complex. This inactivation does not occur at 0 C. The complex could not be reactivated by high concentrations of Mg2+. It is suggested that a phosphorylation-dephosphorylation mechanism may occur in plants, but the phosphatase and kinase are not tightly bound to the complex and are lost on isolation. The complex does not respond in a significant manner to energy charge. The NAD+ to NADH ratio is the principal means of regulation of the complex, NADH being competitive with NAD+ for the dihydrolipoamide component. The CoA to acetyl-CoA ratio is not important in regulation. The castor bean (Ricinus communis L.) proplastid complex is inhibited by the addition of 2 mM MgATP to the assay mixture. The inhibition is immediate, suggesting that phosphorylation of the enzyme is not involved or must be very rapid. Incubation of the complex with 20 mM MgCl2 causes an activation of the complex. Maximum activity is not expressed in this case for 30 minutes. A similar activation can be achieved by preincubating the complex with 1 mM pyruvate. These data suggest that the complex is not fully activated on isolation.