Fuscin, an inhibitor of respiration and oxidative phosphorylation in ox-neck muscle mitochondria

Abstract

1. The effect of fuscin on the mitochondrial oxidation of pyruvate plus malate, of succinate and of ascorbate plus tetramethyl- p-phenylenediamine (TMPD) and on the redox changes of succinate-reducible cytochromes b and c was investigated using tightly-coupled ox-neck muscle mitochondria. 2. Both respiration and oxidative phosphorylation were inhibited by fuscin. Fuscin, at a concentration of 20 nmoles per mg protein gave the following per cent inhibition of oxidative phosphorylation, measured by ADP/O ratio: pyruvate plus malate, 100%; succinate, 22%; ascorbate plus TMPD, 12%. Pre-incubation of the mitochondrial suspension with fuscin was necessary for maximal effect on the State 3 rate, respiratory control index and the ADP/O ratio. 3. Inhibition of State 3 rate induced by ADP with either succinate or ascorbate plus TMPD as substrate but not that of pyruvate plus malate could be released by p-trifluoromethoxy-carbonyl-cyanide-phenylhydrazone, an uncoupler of oxidative phosphorylation. 4. The aerobic steady state reduction of both cytochromes b and c was affected by fuscin, the former being more fuscin-sensitive. 5. The oxidation-reduction cycle of cytochrome c induced by ADP was inhibited by fuscin (29 nmoles per mg protein). At this concentration fuscin also inhibited 40% of the aerobic steady state reduction of cytochrome c and also delayed the time of reaching the aerobic steady state from 5 to 45 s.