Effects of digitonin and tocopherol on bovine heart muscle reduced diphosphopyridine nucleotide- and succinate-cytochrome c reductase and cytochrome c oxidase.

The effects of digitonin and tocopherol on bovine heart reduced diphosphopyridine nucleotideand succinate-cytochrome c reductases and cytochrome c oxidase have been studied with the following results. 1. Low concentrations of digitonin stimulate succinateand DPNH-cytochrome c reductase and cytochrome c oxidase. Higher concentrations of digitonin completely inhibit cytochrome c reductases, while continuing to stimulate cytochrome c oxidases. 2. Inhibition of the reductases is reversed by either dilution or addition of tocopherol. Stimulation of the reductases and oxidase is prevented by tocopherols but is not reversed by either tocopherols or dilution. 3. Of other compounds tested for ability to prevent these digitonin effects, cu-tocopherol polyethylene glycol succinate 1000 and coenzyme Q4 chromanol were effective pith a soluble preparation of DPNH-cytochrome c reductase. In addition to these, ac-tocopheryl succinate and ar-tocopheryl phosphate were effective with a particulate preparation of succinate-cytochrome c reductase. a-Tocopheryl succinate, or-tocopheryl phosphate, a-tocopheryl hydroquinone, and vitamin Dz prevented the stimulation of cytochrome c oxidase. 4. Digitonin stimulation of succinate-cytochrome c reductase is associated with increases in both Vmax and K, (succinate); digitonin inhibition is associated with decreases in both Vm,x and K, (succinate). 5. Reduced with respect to oxidized difference spectra of normal and inhibited preparations indicated succinate reduction of cytochromes (I + a3, b, and cl, and of flavin in the normal enzyme, but almost no reduction of flavin and cytochromes of a digitonin-inhibited enzyme. 6. Digitonin caused almost complete inhibition of enzymatic reduction of ferricyanide by succinate, but only partial