Regulation of Ketogenesis

Abstract

The analysis of the initial-rate kinetics of the liver mitochondrial acetyl-CoA acetyltransferase (acetoacetyl-CoA thiolase) in the direction of acetoacetyl-CoA synthesis under product inhibition was performed.1. Acetyl-CoA acetyltransferase shows a hyperbolic response of reaction velocity to changes in acetyl-CoA concentrations with an apparent K,,, of 0.237 k 0.001 mM.2. CoASH is a (non-competitive) product inhibitor with a Ki, of 22.6 pM and shifts the apparent for acetylCoA to the physiological concentration of this substrate in mitochondria (So,5 = 1.12 mM in the presence of 121 FM CoASH).3. CoASH causes a transformation of the Michaelis-Menten kinetics into initial-rate kinetics with four intermediary plateau regions.4. The product analogue desulpho-CoA triggers a negative cooperativity as to the dependence of the reaction velocity on the acetyl-CoA Concentration.These product effects drastically desensitize the acetyl-CoA acetyltransferase in its reaction velocity response to the acetyl-CoA concentrations and simultaneously extend the substrate dependence range. Thus a control of acetoacetyl-CoA synthesis by the substrate is established over the physiological acetyl-CoA concentration range. We suggest that this control mechanism is the key in establishing the rates of ketogenesis.According to the current concepts, the regulation of ketogenesis is exerted at the site of fatty acid transport into the mitochondria by carnitine acyltransferase [l -31, the effectiveness of which is questionable in starvation [4-81, and by availability of acetyl-CoA. The flux of acetyl-CoA into the citric acid cycle and in the 3-hydroxymethylglutaryl-CoA cycle is mainly determined by the concentration of free oxaloacetate [9,10] which in turn is influenced by changes in the redox states of the mitochondrial pyridine nucleotides [ll] or by N 6 , 0 Zdibutyryladenosine 3 ',5'-monophosphate [12]. Unequivocally these metabolic events render the disposition of acetyl-CoA for ketone body production. However, the mitochondrial acetylCoA concentrations in hepatocytes from fed and starved rats of

Interrelations between C4 Ketogenesis, C5 Ketogenesis, and Anaplerosis in the Perfused Rat Liver

“…Our data extend to C 5 ketogenesis the concept that the mitochondrial 3-ketoacyl-CoA thiolase reaction (Fig. 1, reaction 1) does not allow a net acyl-CoA condensation flux (33). This is shown by the absence of C 5 ketogenesis from propionate (Fig.…”