Triglyceride transfer is required for net cholesteryl ester transfer between lipoproteins in plasma by lipid transfer protein. Evidence for a hetero-exchange transfer mechanism demonstrated by using novel monoclonal antibodies.

In order to investigate the role of lipid transfer protein (LTP) in plasma lipoprotein metabolism, monoclonal antibodies (mAbs) have been raised against LTP isolated from rabbit plasma. mAbs 2-8G and 3-9F inhibited both [3H]cholesteryl ester (CE) and [3H]triglyceride (TG) transfer from low density lipoprotein (LDL) to high density lipoprotein (HDL) mediated by LTP. Although 3-9F cross-reacted with human LTP, 2-8G was species-specific for rabbit LTP. mAb 14-8H inhibited only [3H]TG but not [3H]CE transfer and was cross-reactive with human LTP. mAbs 2-8G and 3-9F interfered with association of LTP with lipid microemulsions, again 2-8G with species specificity, whereas 14-8H did not affect LTP-microemulsion binding. Thus, mAbs 3-9F and 14-8H were used for further study in human plasma. By total inhibition of CE and TG transfer by 3-9F, LTP was shown to be responsible for net mass transfer of neutral lipids between lipoprotein classes in plasma, namely of CE from HDL to very low density lipoprotein (VLDL) and TG from VLDL to LDL and HDL. Selective inhibition of TG transfer by mAb 14-8H was also able to inhibit such net neutral lipid transfer. Such effect of these antibodies was demonstrated more remarkably in the presence of cholesterol esterification. Thus, TG transfer activity of LTP was shown to be required for net CE transfer, suggesting that net neutral lipid transfer in plasma between lipoproteins occurred mainly by a hetero-exchange mechanism. Inhibition of net neutral lipid transfer in plasma did not affect cholesterol esterification occurring predominantly on HDL. Consequently, mAb inhibition of TG transfer in plasma leads to CE accumulation in HDL. It is possible that hyperalphalipoproteinemia may be induced by a mutation in LTP that causes a selective defect in TG transfer activity.