Modifications of citrate and isocitrate metabolism in liver mitochondria of ethanol-fed rats

Abstract

The transports of citrate and isocitrate and the activity of enzymes involved in the metabolism of these substrates were studied in mitochondria isolated from rat alcoholic fatty liver. The reduction of intramitochondrial pyridine-nucleotides following on the addition of citrate with malate and phosphate to mitochondria was assayed with a double-beam spectrophotometer. A marked 79% increase was observed in mitochondria from rats fed ethanol compared to the control group. This increase was not observed with isocitrate. Alteration of transports might contribute to this observed change. The efflux of isocitrate exchanged for extramitochondrial citrate was markedly reduced (−50% in ethanol fed rats), while [ 14C]citrate penetration into malate loaded mitochondria was but slightly inhibited and [ 14C]malate transport was unchanged. Such alterations are similar to that produced by long-chain acyl-CoAs which are elevated in such alcoholic fatty liver. However, acceleration of pyridine-nucleotides reduction following on the addition of citrate is still observed in Triton-disrupted mitochondria. The following intramitochondrial changes may contribute to this acceleration: an increased activity of aconitate hydratase (+32%) and of NAD-linked isocitrate deshydrogenase (+11%), higher levels of mitochondrial pyridine-nucleotides (+21% to 35%). The increased utilization of citrate by mitochondria, along with the loss of activity of the cytosolic ATP-citrate lyase previously described in ethanol-fed rats may depress fatty acid synthesis, in response to the accumulation of triglycerides in the liver cell.