Factors Controlling Ketogenesis by Rat Liver Mitochondria

Factors controlling the rates of ketogenesis by intact rat liver mitochondria have been investigated. High rates of ketone body formation were obtained with (−)-palmitoylcarnitine (20–120 μM) as substrate, but much lower rates were observed when pyruvate (0.33–1.66 mM) or (−)-acetylcarnitine (0.33–1.00 mM) was substrate. Concentrations of CoA-SH, acetyl-CoA, and long-chain acyl-CoA have been determined in mitochondria incubated with each of these substrates in the absence of metabolic inhibitors. In general, rates of ketogenesis increased as CoA-SH levels fell. Although acetyl-CoA concentrations increased in mitochondria incubated in the presence of low concentrations of (−)-palmitoylcarnitine (below 40 μM), they decreased when higher concentrations of (−)-palmitoylcarnitine were employed. This lowering of acetyl-CoA levels occurred concomitantly with an increase in concentrations of long-chain acyl-CoA and a decrease in CoA-SH levels.In soluble mitochondrial fractions obtained after sonication, CoA-SH addition inhibited acetoacetate formation. The ratio of [acetyl-CoA]/[CoA-SH] and the concentrations of CoA-SH were shown to be of greater importance in the regulation of ketogenesis than was the concentration of acetyl-CoA. Additional factors controlling rates of ketogenesis are discussed in relation to data presented. For example, the [acetyl-CoA]/[CoA-SH] ratio was considerably elevated when pyruvate or (−)-acetylcarnitine was substrate, but at such ratios the rates of ketogenesis were far lower than when (−)-palmitoylcarnitine was the substrate. It was calculated that the "apparent Km" of acetoacetyl-CoA for ketone body formation in intact rat liver mitochondria was approximately 10−9 M when (−)-palmitoylcarnitine was the substrate but it was significantly higher when (−)-acetylcarnitine and pyruvate were substrates.