A unified model of esterified cholesterol exchanges between human plasma lipoproteins

Exchanges of esterified cholesterol between human high density lipoproteins (HDL) and very low density lipoproteins (VLDL) and between VLDL and low density lipoproteins (LDL) have been fitted to a mathematical model previously developed to describe exchanges between human HDL and LDL. In all cases the fit of the model predicted exchanges to those measured experimentally was extremely good, thus greatly increasing confidence in the validity of the model. The model assumes that esterified cholesterol exchanges are achieved by means of a transfer protein which interacts with lipoprotein particles from which it picks up and deposits esterified cholesterol. The values generated for the model constants indicated that, given equal concentrations of esterified cholesterol in each fraction, the relative probability that the transfer protein will pick up a molecule of esterified cholesterol in HDL vs. VLDL vs. LDL is in the ratio 28.9:4.65:1. According to the model the transfer protein may 'bind' to lipoproteins. The model predicts that, at physiological lipoprotein concentrations, the proportion of transfer protein bound to HDL will be more than double that which is unbound to lipoprotein and that bound to VLDL will be about one tenth that unbound. The model was unable to detect evidence of the transfer protein binding to LDL.