Effects of acute leptin administration on the differences in proton leak rate in liver mitochondria from ob/ob mice compared to lean controls.

In this investigation, the effects on proton leak of leptin administration to ob/ob mice was measured for liver mitochondria. We and others have shown that proton leak is approximately 3 times greater in liver mitochondria from ob/ob mice compared to lean controls at any given membrane potential. The results are consistent with obese mammals having higher lean mass-specific metabolic rates compared to lean controls. The increase in proton leak rate at any given membrane potential cannot be explained by the presence of free fatty acids associated with mitochondria isolated from the obese animals. The difference in proton leak must therefore represent a real difference in inner membrane permeability. Acute leptin (OB protein) administration restores the liver mitochondrial proton leak rate of ob/ob mice to that of lean controls. There was no effect on proton leak rate of liver mitochondria from sham-treated ob/ob mice. These novel results indicate a role for leptin, either directly or indirectly, in regulating the efficiency of oxidative phosphorylation.

Indirect measurement of mitochondrial proton leak and its application.

In mitochondria, ATP synthesis is coupled to oxygen consumption by the proton electrochemical gradient established across the mitochondrial inner membrane in a process termed oxidative phosphorylation. It has long been known from stoichiometric studies that ATP synthesis is not perfectly coupled to oxygen consumption. The major inefficiency in the system is leakage of protons across the mitochondrial inner membrane driven by the proton electrochemical gradient. The kinetics of the proton leak can be determined indirectly, by measuring the oxygen consumption of mitochondria under non-phosphorylating conditions (plus oligomycin) as a function of the proton electrochemical gradient. This experimental system provides a convenient means to investigate inner membrane permeability to protons and the effect of factors that may effect that permeability. In this paper we review some results from our laboratory of indirect measurement of mitochondrial proton leak and how it has been applied to investigate the effect of aging, obesity and thyroid status on proton leak. The results show that (i) proton leak in isolated liver mitochondria is not significantly different in a comparison of young and old rats, in contrast (ii) there is an apparent increase in proton leak in in situ mitochondria in hepatocytes from old rats when compared to those from young rats, (iii) proton leak in neuronal mitochondria in situ in synaptosomes is not significantly different in young and old rats, (iv) proton leak is greater in isolated liver mitochondria from ob/ob mice compared to lean controls, (v) acute leptin (OB protein) administration restores the increased leak rate in isolated liver mitochondria from ob/ob mice to that of lean controls, (vi) administration of thyroid hormone (T3) increases proton leak in rat muscle mitochondria, and (vii) proton leak in muscle mitochondria is insensitive to the presence of GDP. It is proposed that the experimental system described here for measuring proton leak, is an ideal functional assay for determining whether the novel uncoupling proteins increase inner membrane permeability to protons.