Acetoacetyl-CoA ligase activity in the isolated rat hepatocyte: effects of 25-hydroxycholesterol and high density lipoprotein.

The activity of acetoacetyl-CoA (AcAc-CoA) ligase (E.C.6.2.1.16) in hepatocytes from rats was shown to be the same as the activity in homogenates of their livers. In hepatocytes treated with 25-hydroxycholesterol, AcAc-CoA ligase, 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA) reductase and rates of sterol synthesis were substantially decreased. Hepatocytes treated with high density lipoprotein (HDL) exhibited a 2 to 4 fold induction of HMG-CoA reductase activity; however an accompanying increase in AcAc-CoA ligase activity and the rate of cholesterol synthesis was not observed. We conclude (a) that increases in the activity of HMG-CoA reductase when mediated by HDL in hepatocytes do not result in a corresponding change in the capacity for sterol synthesis and (b) that changes in the activity state of HMG-CoA reductase can be dissociated from that of AcAc-CoA ligase.