The inhibition of NADH oxidase by the lower homologs of coenzyme Q.

The Coenzyme Q homologs having short isoprenoid chains are much less efficient than the higher homologs in restoring NADH oxidation in pentane-extracted lyophilized beef heart mitochondria; they have however high restoring activity for succinate oxidation. The same pattern is observed in pentane extracted submitochondrial particles ETP only if the quinones are added to detergent-treated membranes, showing that in ETP there is a decreased accessibility of the long chain quinones in comparison with the lower homologs. In intact mitochondria and ETP, CoQ₃ inhibits NADH oxidation while leaving succinate oxidation unaffected; the inhibition of NADH oxidation by CoQ₃ is not reversed by serum albumin but is reversed by CoQ₇, particularly when the membrane has been previously “opened” with deoxycholate. CoQ₃ may accept electrons from NADH in cyanide-inhibited ETP, allowing coupling at the first phosphorylation site as shown by the quenching of the fluorescence of atebrine. The mechanism of CoQ₃ inhibition is probably related to its insufficient rate of reoxidation by the following segment of the respiratory chain when it has been reduced by NADH dehydrogenase.