Effects of copper on photosynthetic electron transport systems in spinach chloroplasts

The effects of copper on photosynthetic electron transfer systemsin isolated spinach chloroplasts were studied. Two differentinhibitions were observed. First, copper markedly inhibitedferredoxin-catalyzed reactions such as NADP⁺ photoreduction.The concentration required for 50% inhibition was about 2 µMof cupric sulfate. However, electron flow from reduced 2,6-dichloroindophenol(DCIP) to methyl viologen was not affected. The dissociationconstant between ferredoxin and ferredoxin-NADP⁺ reductase wasunchanged in the presence of 2.5 µM of cupric sulfate.In enzymic reaction systems, the ferredoxin-dependent electronflow from NADPH to cytochrome c was also strongly inhibitedin the presence of cupric sulfate, while DCIP reduction withNADPH as the electron donor was not affected. Second, DCIP photoreductionwas weakly blocked by copper and the lost activity could notbe recovered by adding 1,5-diphenylcarbazide (DPC). It can be concluded that copper directly interacted with ferredoxincausing inhibition of ferredoxin-dependent reactions. Further,copper caused weak inactivation between the oxidizing side ofthe reaction center of photosystem II and the electron donatingsite of DPC. (Received August 8, 1977; )