Factors Controlling Ketogenesis by Rat Liver Mitochondria

Abstract

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.