The metabolism of esterified cholesterol in plasma low density lipoproteins (LDL) has been studied in rabbits. LDL labelled with 3H in the esterified and free cholesterol moieties was isolated from the serum of donar rabbits which has been injected with [3H]mevalonic acid, and subsequently either incubated at 37°C in vitro with unlabelled rabbit serum or unlabelled rabbit lipoprotein fractions, or reinjected into other rabbits. In vitro there was found to be a transfer of 40-60% of the esterified [3H]-cholesterol out of LDL into both the very low density lipoprotein (VLDL) and high density lipoprotein (HDL) fractions which could not be explained in terms of net transfer of esterified cholesterol mass. In the incubations of labelled LDL with either of the other unlabelled lipoprotein fractions, transfers were apparent only if the dialysed 1.21 g/ml infranatant of rabbit serum was also present. The transfer of esterified [3H]cholesterol out of LDL was enhanced when lecithin:cholesterol acyltransferase was active. After reinjecting labelled LDL into other rabbits, it was found that more than half of the esterified [3H]cholesterol removed from the recipient LDL fraction during the first 30 min was not lost from the plasma compartment, but rather was recovered in HDL. There was only minimal in vivo transfer of LDL esterified [3H]cholesterol into VLDL. It has been concluded that in vitro the esterified cholesterol in LDL exchanges with that in both the VLDL and HDL, and that in vivo the esterified cholesterol pools in LDL and HDL may represent parts of a progressively equilibrating plasma pool. © 1979.
