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 synthes is, is activated by light, presumably under redox regulation. To obtain evi dence of redox regulation in vivo, the activity of ACCase was examined in p ea chloroplasts isolated from plants kept in darkness (dark-ACCase) or afte r 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 f or light-ACCase and dark-ACCase, but the activity of light-ACCase in the ab sence of DTT was approx. 3-fold that of dark-ACCase. The light-ACCase and d ark-ACCase were activated approx. 2-fold and 6-fold by DTT respectively, in dicating that light-ACCase was in a much more reduced, active form than the dark-ACCase. This is the first demonstration of the light-dependent reduct ion of ACCase in vivo. Measurement of the activities of ACCase, carboxyl-tr ansferase and biotin carboxylase in the presence and absence of DTT, and th e thiol-oxidizing agent, 5,5'-dithiobis-(2-nitrobenzoic) acid, revealed tha t the carboxyltransferase reaction, but not the biotin carboxylase reaction , was redox-regulated. The cysteine residue(s) responsible for redox regula tion probably reside on the carboxyltransferase component. Measurement of t he pH dependence of biotin carboxylase and carboxyltransferase activities i n the ACCase suggested that both components affect the activity of ACCase i n vivo at a physiological pH range. These results suggest that the activati on of ACCase by light is caused partly by the pi-I-dependent activation of two components and by the reductive activation of carboxyltransferase.