Abstract
Summary Two mitochondrial forms of acetoacetyl-CoA thiolases (A and B) have been obtained from rat liver. A kinetic analysis of thiolytic activities revealed for both enzymes marked inhibition by the substrate acetoacetyl-CoA and negative cooperativity with respect to CoA. In the reverse reaction (condensation) different kinetic properties were evident: The affinity of enzyme A for acetyl-CoA is lowered by acetoacetyl-CoA. A possible allosteric control of enzyme A by acetoacetyl-CoA is deduced from negative cooperativity with respect to binding of acetyl-CoA, depending on the concentrations of acetoacetyl-CoA. On the contrary, enzyme B shows simple Michaelis-Menten kinetics with acetyl-CoA that are not altered by acetoacetyl-CoA. A possible regulatory role of acetoacetyl-CoA thiolase A in ketogenesis is deduced from the above data.
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