Regulation of lipid transfer between lipoproteins by an endogenous plasma protein: selective inhibition among lipoprotein classes.

Lipid transfer protein (LTP) remodels plasma lipoproteins by promoting mass transfers of cholesteryl ester (CE) and triglyceride between lipoproteins. We have investigated the capacity of an additional plasma protein, lipid transfer inhibitor protein (LTIP) to modify the functional activity of LTP in a complex mixture of lipoproteins. Transfer assays containing isolated LTP, LTIP, and the three major lipoprotein classes, and assays with intact human plasma supplemented with exogenous LTIP were used. In both assays, the inhibition of CE transfer by LTIP varied markedly depending on the lipoproteins involved and was dependent on LTIP concentration. Inhibition of lipid transfer between a given pair of lipoproteins was similar. However, between lipoprotein pairs the extent of inhibition was very different, varying up to 7-fold. Inhibition followed the order of very low density lipoprotein (VLDL)-low density lipoprotein (LDL) transfers > LDL-high density lipoprotein (HDL) transfers > VLDL-HDL transfers. Consistent with the preferential inhibition of transfer events involving LDL, LTIP was shown by gel filtration studies to associate primarily with LDL in plasma. The addition of LTIP to native plasma stimulated the LTP-mediated efflux of CE from HDL to VLDL; this occurred at the expense of LDL CE depletion. Thus, LTIP alters the pattern of lipid transfer reactions in plasma by uniquely affecting the individual transfer events mediated by LTP. By preferentially diminishing transfer events involving LDL, especially those between VLDL and LDL, LTIP enhances the ability of LTP to remove CE from HDL, and thus alters HDL metabolism.