The activity of an esterified cholesterol transferring factor in human and rat serum

In incubations containing mixtures of lipoproteins isolated either from humans or from rats, the addition of human lipoproteins-free serum (the dialysed 1.25 g/ml infranatant solution) was more than five times more effective than that of the rat in promoting the transfer of esterified [3H]cholesterol from high density lipoproteins to very low density lipoproteins, regardless of the species origin of the lipoproteins, implying the existence of an esterified cholesterol transferring factor of much greater activity in human than in rat serum.