The relationship between electron flux and the redox poise of the quinone pool in plant mitochondria. Interplay between quinol-oxidizing and quinone-reducing pathways.

The dependence of electron flux through quinone-reducing and quinol-oxidizing pathways on the redox state of the ubiquinone (Q) pool was investigated in plant mitochondria isolated from potato (Solanum tuberosum cv. Bintje, fresh tissue and callus), sweet potato (Ipomoea batatas) and Arum italicum. We have determined the redox state of the Q pool with two different methods, the Q-electrode and Q-extraction techniques. Although results from the two techniques agree well, in all tissues tested (with the exception of fresh potato) an inactive pool of QH2 was detected by the extraction technique that was not observed with the electrode. In potato callus mitochondria, an inactive Q pool was also found. An advantage of the extraction method is that it permits determination of the Q redox state in the presence of substances that interfere with the Q-electrode, such as benzohydroxamate and NADH. We have studied the relation between rate and Q redox state for both quinol-oxidizing and quinone-reducing pathways under a variety of metabolic conditions including state 3, state 4, in the presence of myxothiazol, and benzohydroxamate. Under state 4 conditions or in the presence of myxothiazol, a non-linear dependence of the rate of respiration on the Q-redox state was observed in potato callus mitochondria and in sweet potato mitochondria. The addition of benzohydroxamate, under state 4 conditions, removed this non-linearity confirming that it is due to activity of the cyanide-resistant pathway. The relation between rate and Q redox state for the external NADH dehydrogenase in potato callus mitochondria was found to differ from that of succinate dehydrogenase. It is suggested that the oxidation of cytoplasmic NADH in vivo uses the cyanide-resistant pathway more than the pathway involving the oxidation of succinate. A model is used to predict the kinetic behaviour of the respiratory network. It is shown that titrations with inhibitors of the alternative oxidase cannot be used to demonstrate a pure overflow function of the alternative oxidase.