INVITRO EXCHANGES OF ESTERIFIED CHOLESTEROL BETWEEN SERUM LIPOPROTEIN FRACTIONS - STUDIES OF HUMANS AND RABBITS

Lipoproteins labeled with 3H in the free- and esterified-cholesterol moieties were isolated from the serum of rabbits that had been injected 14 hr before with 3H-mevalonic acid and from human serum that had been incubated with 3H-cholesterol for 6 hr at 37°C. Each of the labeled lipoprotein fractions of each species was subsequently incubated in vitro at 37°C with unlabeled lipoprotein fractions isolated from the same species in the presence of an inhibitor of lecithin:cholesterol acyltransferase (LCAT) activity. Incubations contained either the lipoprotein-free (the dialysed 1.21 g/ml infranate) fraction of serum or an equivalent volume of buffer. In incubations of both rabbit and human lipoproteins there was a significant bidirectional transfer of esterified 3H-cholesterol between each pair of lipoproteins so long as the 1.21 g/ml infranate was present, but in its absence there were no transfers. A minimum estimate of the rate at which esterified cholesterol exchanged between human lipoproteins was obtained; expressed as μmole/liter serum/hr, it was 30-166 between very low density lipoproteins (VLDL) and low density lipoproteins (LDL) 31-156 between VLDL and high density lipoproteins (HDL) and 97-140 between LDL and HDL. In subsequent experiments, labeled human VLDL was incubated with unlabeled human LDL in the presence of the 1.21 g/ml infranate isolated from the serum of different species. In terms of promoting a transfer of esterified 3H-cholesterol into the LDL, the rabbit 1.21 g/ml infranate proved to be more effective than that of the human, but the 1.21 g/ml infranate of rat serum was totally ineffective. It has been concluded that a process of esterified-cholesterol exchange between human serum lipoproteins may be rapid enough to have the potential of major physiologic importance. © 1979.