Conditions that may result in (de-)phosphorylation of hepatic acyl-CoA:cholesterol acyltransferase result also in modulation of substrate supply in vitro.

The present experiments were designed to study intervesicular transfer of cholesterol in rat liver microsomal fraction and modulation of the activity of acyl-CoA:cholesterol acyltransferase (ACAT) under conditions that are expected to result in the covalent modification (phosphorylation/dephosphorylation) of the enzyme. Preincubation of rat liver microsomal fraction followed by assay of ACAT showed a time-dependent increase in activity. This rate was temperature-dependent. Preincubation in the presence of cholesterol/phospholipid liposomes resulted in a time-dependent transfer of cholesterol from liposomal to the microsomal vesicles and in an increase in the rate of ACAT change owing to the preincubation. Both these rates were dependent on liposomal cholesterol concentration and on temperature. The presence of cytosol in the preincubation mixture increased the rate of change of ACAT activity in the absence or in the presence of cholesterol/phospholipid liposomes. In the latter case the presence of cytosol also increased the rate of transfer of cholesterol from liposomal to the microsomal vesicles. Activation energies of the rate of this transfer and of the rate of increase of ACAT activity were similar in the presence and in the absence of cytosol. Both in the absence and in the presence of cytosol, the presence of NaF (50 mM) in the preincubation mixture considerably decreased the rate of transfer of cholesterol from liposomal to microsomal vesicles and the rate of increase of ACAT activity. The presence of Mg2+ in the preincubation mixture produced no effect on the rate of transfer of cholesterol from liposomal to the microsomal vesicles, although under most conditions it decreased the rate of increase of ACAT activity caused by the preincubation. These results are discussed in relation to the molecular mechanism involved in this intervesicular transfer of cholesterol and to the modulation of ACAT activity by substrate supply, and also in relation to the hypothesis that ACAT activity can be modulated by a mechanism involving the phosphorylation/dephosphorylation of the enzyme.