On the mechanism of the chemical modification of the mitochondrial acetyl-CoA acetyltransferase by coenzyme A

The liver mitochondrial acetyl-CoA acetyltransferase (acetyl-CoA:acetyl-CoA C-acetyltransferase, EC 2.3.1.9), is involved in ketone body synthesis. The enzyme can be chemically modified and inactivated by CoASH and also by CoASH-disulfides provided glutathione is present. The unmodified enzyme shows in its denatured state 7.95 +/- 0.44 sulfhydryl groups per enzyme and in its native state 3.92 +/- 0.34 sulfhydryl groups which react with Ellmann's reagent. The modified enzyme reveals in its native state also 4.07 +/- 0.25 sulfhydryl groups per enzyme, but in its denatured state 9.10 +/- 0.51 sulfhydryl groups could be detected. Approximately four sulfhydryl groups per enzyme, unmodified or modified, can be alkylated by iodoacetamide. These results prove for each subunit the existence of two sulfhydryl groups and suggest the existence of two disulfide bridges. The CoASH modification, which should proceed at one of these disulfide groups, prevents subsequent acetylation of the enzyme and is drastically reduced in the iodoacetamide-alkylated enzyme. In the demodification of the modified enzyme, the CoASH is set free as a mixed disulfide with glutathione.