Nucleotide effects on liver and muscle mitochondrial non-phosphorylating respiration and membrane potential.

Uncoupling protein-1 homologs are hypothesized to mediate mitochondrial proton leak. To test this hypothesis, we determined the effects of ATP and other nucleotides on liver and skeletal muscle mitochondrial non-phosphorylating respiration (VO(2)), membrane potential, FCCP-stimulated respiratory control ratios, and swelling. Neither ATP nor CTP affected liver or muscle proton leak, but both inhibited the respiratory chain. Unexpectedly, CMP stimulated liver proton leak (EC(50) approximately 4.4+/-0.5 mM). Using CMP chromatography, we identified two proteins (M(r)=31.2 and 32.6 kDa) from liver mitochondria that are similar in size to members of the mitochondrial carrier protein family. We conclude (a) liver and muscle mitochondrial proton leak is insensitive to ATP and CTP, and (b) CMP activates a leak in liver mitochondria. The CMP-inducible leak may be mediated by a 30-32 kDa protein. Based on the high concentrations required, CMP is unlikely to be a physiologically important leak regulator. Nonetheless, our results show that tissues other than brown fat have inducible leaks that may be protein-mediated.