title: Cytochrome function in the cyclic electron transport pathway of chloroplasts.
abstract: Flash excitation of isolated intact chloroplasts promoted absorbance transients corresponding to the electrochromic effect (P-518) and the alpha-bands of cytochrome b6 and cytochrome f. Under conditions supporting coupled cyclic electron flow, the oxidation of cytochrome b6 and the reduction of cytochrome f had relaxation half-times of 15 and 17 ms, respectively. Optimal poising of cyclic electron flow, achieved by addition of 0.1 microM 3-(3,4-dichlorophenyl)-1,1-dimethylurea, increased phosphorylation of endogenous ADP and prolonged these relaxation times. The presence of NH4Cl, or monensin plus NaCl, decreased the half-times for cytochrome relaxation to approximately 2 ms. Uncouplers also revealed the presence of a slow rise component in the electrochromic absorption shift with formation half-time of about 2 ms. Ths inhibitors of cyclic phosphorylation antimycin and 2,5-dibromo-3-methyl-6-isoprophy-p-benzoquinone abolished the slow rise in the electrochromic shift and prolonged the uncoupled relaxation times of cytochromes b6 and f by factors of ten or more. These observations indicate that cytochrome b6, plastoquinone and cytochrome f participated in a coupled electron transport process responsible for cyclic phosphorylation in intact chloroplasts. Estimations of cyclic phosphorylation rates from 40 to 120 mumol ATP/mg chlorophyll per h suggest that this process can provide a substantial fraction of the ATP needed for CO2 fixation.
mesh_terms: D000968:Antimycin A / Q000494:pharmacology; D002736:Chloroplasts / Q000187:drug effects / Q000378:metabolism*; D003580:Cytochromes / Q000378:metabolism; D003624:Darkness; D004237:Diuron / Q000494:pharmacology; D004579:Electron Transport; D007700:Kinetics; D008027:Light; D010785:Photophosphorylation / Q000187:drug effects; D010788:Photosynthesis / Q000187:drug effects; D010944:Plants; D013053:Spectrophotometry
chemical_list: D003580:Cytochromes; D000968:Antimycin A; D004237:Diuron