Light-dependent changes in redox status of the plastidic acetyl-CoA carboxylase and its regulatory component.

Plastidic acetyl-CoA carboxylase (ACCase; EC 6.4.1.2), which catalyses the synthesis of malonyl-CoA and is the regulatory enzyme of fatty acid synthesis, is activated by light, presumably under redox regulation. To obtain evidence of redox regulation in vivo, the activity of ACCase was examined in pea chloroplasts isolated from plants kept in darkness (dark-ACCase) or after exposure to light for 1 h (light-ACCase) in the presence or absence of a thiol-reducing agent, dithiothreitol (DTT). The protein level was similar for light-ACCase and dark-ACCase, but the activity of light-ACCase in the absence of DTT was approx. 3-fold that of dark-ACCase. The light-ACCase and dark-ACCase were activated approx. 2-fold and 6-fold by DTT respectively, indicating that light-ACCase was in a much more reduced, active form than the dark-ACCase. This is the first demonstration of the light-dependent reduction of ACCase in vivo. Measurement of the activities of ACCase, carboxyltransferase and biotin carboxylase in the presence and absence of DTT, and the thiol-oxidizing agent, 5, 5'-dithiobis-(2-nitrobenzoic) acid, revealed that the carboxyltransferase reaction, but not the biotin carboxylase reaction, was redox-regulated. The cysteine residue(s) responsible for redox regulation probably reside on the carboxyltransferase component. Measurement of the pH dependence of biotin carboxylase and carboxyltransferase activities in the ACCase suggested that both components affect the activity of ACCase in vivo at a physiological pH range. These results suggest that the activation of ACCase by light is caused partly by the pH-dependent activation of two components and by the reductive activation of carboxyltransferase.