The effect of digitonin on the respiratory chain enzymes of a heart-muscle preparation

Abstract

Summary 1. Differential inactivation by digitonin of succinate oxidase and NADH oxidase, as well as succinate-cytochrome c reductase and NADH-cytochrome c reductase was found in a heart-muscle preparation. 2. The degree of inactivation of the enzymic activities depends on the digito-nin/protein ratio rather than on the digitonin concentration. 3. For a digitonin/protein ratio of 1, the activities of succinate oxidase and NADH oxidase are 40 and 80% of the control, respectively. Similarly for a digito-nin/protein ratio of 5, the activities of succinate-cytochrome c and of NADH-cytochrome c reductase are 20 and 80% of the control, respectively. 4. Succinate dehydrogenase is inactivated by digitonin, but NADH dehydrogenase is not. 5. Cytochrome oxidase is not inactivated by digitonin when ascorbate-cytochrome c is used as the substrate, but is inactivated by 70% when ascorbate-tetra-methyl- p -phenylenediamine (TMPD) is the substrate. 6. Digitonin-inactivated NADH oxidase is fully restored by TMPD-cytochrome c and partly by cytochrome c. However, digitonin-inactivated succinate oxidase is restored by TMPD-cytochrome c by only 40%. 7. The results suggest that differential digitonin inactivation of succinate oxidation and NADH oxidation is caused by inhibition of electron transfer in the succinate chain at two sites, probably in the flavoprotein and cytochrome c regions, whereas NADH oxidation is inhibited in the cytochrome c region only.