Effects of copper on photosynthetic electron transport systems in spinach chloroplasts

The effects of copper on photosynthetic electron transfer systems in isolated spinach chloroplasts were studied. Two different inhibitions were observed. First, copper markedly inhibited ferredoxin-catalyzed reactions such as NADP+ photoreduction. The concentration required for 50% inhibition was about 2 μM of cupric sulfate. However, electron flow from reduced 2,6-dichloroindophenol (DCIP) to methyl viologen was not affected. The dissociation constant between ferredoxin and ferredoxin-NADP+ reductase was unchanged in the presence of 2.5 μM of cupric sulfate. In enzymic reaction systems, the ferredoxin-dependent electron flow from NADPH to cytochrome c was also strongly inhibited in the presence of cupric sulfate, while DCIP reduction with NADPH as the electron donor was not affected. Second, DCIP photoreduction was weakly blocked by copper and the lost activity could not be recovered by adding 1,5-diphenylcarbazide (DPC). It can be concluded that copper directly interacted with ferredoxin causing inhibition of ferredoxin-dependent reactions. Further, copper caused weak inactivation between the oxidizing side of the reaction center of photosystem II and the electron donating site of DPC.