title: Regulation of ketogenesis. Mitochondrial acetyl-CoA acetyltransferase from rat liver: initial-rate kinetics in the presence of the product CoASH reveal intermediary plateau regions.
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 Km of 0.237 +/- 0.001 mM. 2. CoASH is a (non-competitive) product inhibitor with a Kis of 22.6 microM and shifts the apparent Km for acetyl-CoA to the physiological concentration of this substrate in mitochondria (S0.5 = 1.12 mM in the presence of 121 microM 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.
mesh_terms: D000101:Acetyl-CoA C-Acetyltransferase / Q000378:metabolism*; D000123:Acetyltransferases / Q000378:metabolism*; D000818:Animals; D003065:Coenzyme A / Q000031:analogs & derivatives / Q000494:pharmacology; D007700:Kinetics; D008930:Mitochondria, Liver / Q000201:enzymology*; D011485:Protein Binding; D051381:Rats
chemical_list: C035999:desulfo-coenzyme A; D000123:Acetyltransferases; D000101:Acetyl-CoA C-Acetyltransferase; D003065:Coenzyme A