Regulation of fatty acid oxidation in rat brain mitochondria: Inhibition of high rates of palmitate oxidation by ADP

Abstract

Regulation of oxidation of [1- 14C]palmitate in rat brain mitochondria has been investigated in purified mitochondria of nonsynaptic origin prepared by use of a Ficoll/sucrose density gradient. The mitochondrial preparation contained considerable Mg 2+-ATPase activity, but was virtually free of contamination with nonmitochondrial fractions. Palmitate oxidation was inhibited by increasing the concentration of ATP in the assay system to near-physiological levels (2 m m), and the inhibition at 2 or 4 m m ATP was analyzed by comparing it with palmitate oxidation at near-maximal rates with low levels of ATP (0.5 or 1 m m). Inhibition was increased by the addition of ADP or by increasing the concentration of Mg 2+ in the assay system, whereas inhibition was decreased by decreasing the concentration of mitochondrial protein or L-carnitine in the assay system. Increasing CoA concentration also had a deinhibitory effect. With 0.5 or 1 m m ATP, however, neither inhibition by added ADP nor protein concentration-dependent inhibition was observed, and the rate of oxidation was saturated with increasing concentrations of Mg 2+, l-carnitine, or CoA. These results indicated that ADP was involved in the inhibition of high rates of palmitate oxidation in the presence of sufficient ATP and l-carnitine. The inhibitory effect of increasing the concentration of mitochondrial protein could be explained by the enhanced amounts of ADP present in the preparation; similarly, increased concentrations of Mg 2+ would provide higher levels of ADP by stimulating the Mg 2+-ATPase reaction. We discuss the possibility that the transport of ADP across the inner membrane of brain mitochondria is coupled to the inhibition of palmitate oxidation.