Diaminodurene-induced plastocyanin dependent oxygen uptake and its relation to photophosphorylation in isolated lettuce chloroplasts. A comparison of the systems using either water or ascorbate as the electron donors.

Abstract

1 Addition of diaminodurene induced in lettuce chloroplasts an oxygen uptake, which as a typical pseudocyclic electron transport was inhibited by 3-(3,4-dichlorophenol)-1,1-dimethylurea and stimulated by uncouplers. ATP formation was coupled to this electron transport from water through diaminodurene to oxygen, resulting in a P/2e value close to 1. 2 Diaminodurene induced also an oxygen uptake in the presence of dichlorophenol-dimethylurea and ascorbate, which was not affected by uncouplers. The rate of the oxygen uptake in this system changed as a function of diaminodurene concentration and was between 3 to 15 times faster than the oxygen uptake with water as the electron donor. The rate of ATP formation in this system was also dependent on the concentration of diaminodurene, but was much slower than the oxygen uptake, the resulting P/2e value being about 0.18. 3 Under argon a cyclic dichlorophenol-dimethylurea-resistant diaminodurene-dependent ATP formation was obtained. The rates of this cyclic phosphorylation were similar to those observed under air in the presence of ascorbate, although in the last case a ten-fold faster electron transport to oxygen accompanied the phosphorylation. 4 Sonication of the chloroplasts resulted in 85% inhibition of the diaminodurene-induced oxygen uptake when either water or ascorbate served as the electron donors. The inhibition was relieved by the addition of plastocyanin, which restored the original rates of oxygen uptake with both electron donors. 5 It is proposed that a free radical of diaminodurene, which is stabilized by interaction with a chloroplast component, possibly the ferredoxin reducing substance, is the compound acting as an autoxidizable electron acceptor upon addition of diaminodurene to lettuce chloroplasts.