Inhibition of photosystem II activity by Cu(++) ion. Choice of buffer and reagent is critical.

Cupric ion (Cu(++)) inhibits the rate of photosystem II electron transport and the intensity of the variable part of chl a fluorescence in isolated chloroplast thylakoids. The inhibition is markedly dependent on the nature of the buffer used in the assay medium. In MES and HEPES buffers, complete inhibition of photosystem II occurs at 30 μM of Cu(++), while in Tricine no inhibition occurred even at 200 μM Cu(++). In other buffers used (TES, Phosphate, Tris), the efficacy of Cu(++) inhibition is intermediate. The calculated binding constants are found to correspond to the observed I50 values for the six buffers used. It is concluded that the previous reports on copper inhibition, where buffers have been used indiscriminately should be reconsidered. Certain reagents such as hydroxylamine, ascorbate and diphenyl carbazide, which react with Cu(++), should be avoided.