Ubiquinone redox behavior in plant mitochondria during electron transport

The redox poise of the ubiquinone pool during electron transfer in isolated soybean mitochondria has been compared using two different procedures: the rapid organic extraction of ubiquinone followed by quantification of the oxidized and reduced forms using high-pressure liquid chromatography and an electrochemical technique that measures ubiquinone reduction voltametrically. The goal of these studies was to rigorously test the use of the voltametric technique to monitor the redox status of ubiquinone during the course of mitochondrial electron transfer. The linear relationship between the two methods confirms the reliability of the data obtained with the voltametric technique; however, redox inactive pools of ubiquinone were detected with the HPLC technique. We also quantified the absolute amounts of the ubiquinone homologues ubiquinone-9 and ubiquinone-10 in mitochondria isolated from different soybean tissues and compared their behavior during electron transfer in the presence and absence of pyruvate, an allosteric effector of the cyanide-resistant electron transfer pathway. Both homologues belong to a common redox-active pool in the inner mitochondrial membrane. The results indicate that ubiquinone can be a limiting component of electron transfer through the cyanide-resistant pathway, particularly in roots where its concentration is much lower than in cotyledons.