The Respiratory Chain of Plant Mitochondria

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Abstract
Addition of 90 micromolar reduced nicotinamide adenine dinucleotide (NADH) in the presence of cyanide to a suspension of aerobic mung bean (Phaseolus aureus) mitochondria depleted with ADP and uncoupler gives a cycle of reduction of electron transport carriers followed by reoxidation, as NADH is oxidized to NAD+ through the cyanide-insensitive, alternate oxidase by excess oxygen in the reaction medium. Under these conditions, cytochrome b553 and the nonfluorescent, high potential flavoprotein Fpha of the plant respiratory chain become completely reduced with half-times of 2.5 to 2.8 seconds for both components. Reoxidation of flavoprotein Fpha on exhaustion of NADH is more rapid than that of cytochrome b553. There is a lag of 1.5 seconds after NADH addition before any reduction of ubiquinone can be observed, whereas there is no lag perceptible in the reduction of flavoprotein Fpha and cytochrome b553. The half-time for ubiquinone reduction is 4.5 seconds, and the extent of reduction is 90% or greater. About 30% of cytochrome b557 is reduced under these conditions with a half-time of 10 seconds; both cytochrome b562 and the fluorescent, high potential flavoprotein Fphf show little, if any, reduction. The two cytochromes c in these mitochondria, c547 and c549, are reduced in synchrony with a half-time of 0.8 second. These two components are already 60% reduced in the presence of cyanide but absence of substrate, and they become completely reduced on addition of NADH. These results indicated that reducing equivalents enter the respiratory chain from exogenous NADH at flavoprotein Fpha and are rapidly transported through cytochrome b553 to the cytochromes c; once the latter are completely reduced, reduction of ubiquinone begins. Ubiquinone appears to act as a storage pool for reducing equivalents entering the respiratory chain on the substrate side of coupling site 2. It is suggested that flavoprotein Fpha and cytochrome b553 together may act as the branching point in the plant respiratory chain from which forward electron transport can take place to oxygen through the cytochrome chain via cytochrome oxidase, or to oxygen through the alternate, cyanide-insensitive oxidase via the fluorescent, high potential flavoprotein Fphf.