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Articles

The Respiratory Chain of Plant Mitochondria

V. Reaction of Reduced Cytochromes a and a₃ in Mung Bean Mitochondria with Oxygen in the Presence of Cyanide

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

The half-times of oxidation by oxygen pulses of reduced cytochromes a and a₃ in mung bean mitochondria made anaerobic with succinate have been measured by means of a rapid mixing flow apparatus coupled to a dual wave length spectrophotometer in the presence and absence of cyanide. The absorbance changes at 438 to 455 millimicrons and 603 to 620 millimicrons are suitable for recording the time course of cytochrome a oxidation; the half-time is 2.0 milliseconds at 24 Celsius. This half-time does not change over the range 0 to 300 µM KCN, but the fraction of cytochrome a oxidized falls to a limiting value of 0.3 at the higher cyanide concentrations. The absorbance changes at 445 to 455 millimicrons record the time course of both cytochrome a and cytochrome a₃ oxidation; the former contributes 60% of the absorbance change and the latter 40%. The half-time for a₃ oxidation is calculated as 0.9 milliseconds at 24 Celsius. This half-time increases slightly to 1.3 milliseconds at 300 µM KCN. Reduced cytochrome a₃, whether uncomplexed or complexed with cyanide, becomes fully oxidized. The dissociation constant for the reduced cytochrome a₃-cyanide complex is estimated to be 30 µM, whereas that for the oxidized a₃-cyanide complex which inhibits electron transport is estimated to be 2 µM. This suggests two different binding sites for cyanide on the reduced and oxidized forms of cytochrome a₃. The fact that a limiting fraction of reduced cytochrome a can be oxidized at high cyanide concentrations implies that there is no interference by cyanide with electron transport from a to a₃, if cyanide remains bound to the site it occupies on reduced a₃ after this carrier becomes oxidized on reaction with molecular oxygen. Rearrangement of cyanide from this noninhibitory site to the inhibitory site occurs rapidly enough to compete with cytochrome a oxidation. The half-time for the rearrangement is calculated to be 0.9 milliseconds.