Photophosphorylation as mediated by vitamin K₃ in broken spinach chloroplasts

The effects of DCMU on menadione or menadione sodium bisulphite (MSB) mediated photophosphorylation in broken chloroplasts kept in a Tricine or in a HEPES buffer at either high (131 W m-2) or low irradiance (57 W m-2) in combination with NaN3, KCN, under white, red or far red light, were studied. Some combinations of the factors tested created a system where DCMU inhibited cyclic ATP formation, whereas other combinations created a system insensitive to DCMU or even with a stimulation of ATP formation by presence of the inhibitor. When menadione or MSB acted as a terminal electron acceptor, oxygen was consumed on their reoxidation. The results indicate that this process may occur with oxygen being released by the splitting of water, the main oxidant. The data demonstrate a difference between lipophilic and hydrophilic forms of vitamin K3 in their capacity to mediate photophosphorylation. The results emphasize the importance of caution when selecting buffers, light intensities and cofactors as well as combinations of such factors for experiments on photophosphorylation on chloroplasts.