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The Respiratory Chain of Plant Mitochondria

IV. Oxidation Rates of the Respiratory Carriers of Mung Bean Mitochondria in the Presence of Cyanide

¹ Johnson Research Foundation, University of Pennsylvania, Philadelphia, Pennsylvania 19104

The half-time for oxidation of cytochrome b₅₅₇ in mitochondria from etiolated mung bean (Phaseolus aureus) hypocotyls is 5.8 milliseconds at 24 Celsius in the absence or presence of 0.3 mM KCN, when the oxidation is carried out by injecting a small amount of oxygenated medium into a suspension of mitochondria made anaerobic in the presence of succinate plus malonate. Since oxygen is consumed by the alternate, cyanide-insensitive respiratory pathway of these mitochondria, cycles of oxidation and reduction can be obtained with the oxygen pulses when cyanide is present. Reduced cytochromes (a + a₃) also become oxidized at nearly the uninhibited rate under these conditions, a₃ completely and a partially. The half-time for oxidation of c₅₄₇ is also unaffected by 0.3 mM KCN, but c₅₄₉ has a half-time equal to that of c₅₄₇ in the presence of KCN, compared to the shorter one observed in the absence of inhibitor. The maximum extent of oxidation of the cytochromes c is about 70% in the presence of 0.3 mM KCN; this oxidation is rapidly followed by an extensive reduction which is synchronous with the reduction of cytochrome a observed under the same conditions. In the presence of cyanide, it appears likely that the cytochromes c and b₅₅₇ are oxidized by cytochrome oxidase in oxygen pulse experiments, rather than by the alternate oxidase. The oxidation of cytochrome b₅₅₃ is partially inhibited by KCN, but complete oxidation is attained in the aerobic steady state with excess oxygen. If the oxygen pulse experiment is carried out in the presence of sufficient malonate so that entry of reducing equivalents into the respiratory chain occurs at a rate negligible compared to inter-carrier electron transport, the half-time for flavoprotein oxidation is unaffected by 0.3 mM KCN while that for ubiquinone oxidation is but 2-fold larger. The observed net oxidation rate of these two carriers in mung bean mitochondria is more sensitive to the entry rate of reducing equivalents, as set by succinate concentration and malonate to succinate ratio, then it is in skunk cabbage (Symplocarpus foetidus) mitochondria. These observations are interpreted in terms of a respiratory carrier Y, placed between flavoprotein plus ubiquinone and the cytochromes, which is the fork in the split respiratory pathway to the two terminal oxidases and which has lower electron transport capacity in mung bean mitochondria than in skunk cabbage mitochondria.