Cholesterol exchange between microsomal, mitochondrial and erythrocyte membranes and its enhancement by cytosol.

1. Cholesterol exchanges between isolated rat liver microsomes and mitochondria and between erythrocytes and microsomes or mitochondria during incubation in vitro. The exchange process is temperature dependent and is no accompanied by a net movement of sterol. 2. cholesterol exchange between the membranes was enhanced by the addition of 105 000 x g supernatant fraction (S105) from rat liver. The degree to which sterol exchange was enhanced was dependent on the amount of this supernatant fraction present in the incubation. 3. enhancement of sterol exchange was not observed with heated S105 fraction, but activity was retained after dialysis or aging at 10 degrees C; these results suggest the presence of a cholesterol-exchange protein in the cytosol from rat liver.