Properties of the citrate transporter in rat heart: implications for regulation of glycolysis by cytosolic citrate

Abstract

The efflux of [14C]citrate from rat heart mitochondria was significantly greater with L-malate as the extramitochondrial substrate as compared with [12C]citrate, isocitrate or phosphoenolpyruvate. The concentration of L-malate required for half-maximal rate of efflux of citrate was 0.45 mM and the maximum velocity was 0.36 nmol min-1 mg-1 mitochondrial protein at 23 degrees C. This citrate transporter was inhibited by 1,2,3-benzenetricarboxylate and palmitoyl-CoA but not to the same extent as these compounds inhibit the tricarboxylate carrier in rat liver mitochondria. The apparent inability of these mitochondria to transport citrate in the inward direction necessitates the presence of a cytosolic citrate removal pathway. We propose that the enzymes of this pathway in rat heart could be ATP citrate (pro-3S)-lyase (EC 4.1.3.a) and carnitine acetyltransferase (EC 2.3.1.7), both of which we demonstrate to have adequate activity in both the fed and fasted state. An hypothesis has been put forward to account for the inhibition of rat heart phosphofructokinase by citrate in the fasted state incorporating these properties of the citrate transporter and ATP citrate (pro-3S)-lyase.