Evidence of the existence of a high-density lipoprotein transformation factor in pig and rabbit plasma

Experiments have been conducted to elucidate the incubation conditions necessary for the transformation of pig high-density lipoproteins (HDL) from particles resembling the HDL3 subfraction of human plasma into particles resembling human HDL2. All incubations were performed at 37 degrees C for 24 h in the presence of 0.002 M p-chloromercuriphenylsulphonic acid, a chemical inhibitor of lecithin-cholesterol acyltransferase. There was no transformation when pig HDL (1.09 less than d less than 1.21 g/ml) was incubated either in isolation or in the presence of bovine serum albumin. In addition, there was no transformation when the 1.25 g/ml infranatant of pig plasma was added to the incubation. In the presence of the 1.25 g/ml infranatant of rabbit plasma, by contrast, a significant proportion of the pig HDL was transformed into HDL2. The possibility that this result may have been the consequence of the lipid-transfer activity in rabbit plasma was excluded by the observation that a partially purified preparation of rabbit lipid-transfer protein was ineffective in promoting the HDL transformation. Experiments were also performed with the 1.09 g/ml infranatant of pig plasma. This fraction contained HDL and the plasma proteins but no other lipoprotein. In contrast to the absence of change in incubations of a mixture containing isolated pig HDL (1.09 less than d less than 1.21 g/ml) and the 1.25 g/ml infranatant of pig plasma, during incubation of the 1.09 g/ml infranatant there was an obvious formation of HDL2. It has been concluded that there exists in plasma an HDL-transformation factor which is distinct from both lecithin-cholesterol acyltransferase and the lipid-transfer protein. This factor was detectable in the 1.25 g/ml infranatant of rabbit plasma and in the 1.09 g/ml infranatant of pig plasma. As the factor was not detected in isolated pig HDL or in the 1.25 g/ml infranatant of pig plasma, the 1.21-1.25 g/ml fraction of plasma may be a useful starting point for further investigation of this factor.