Activation of human plasma lipid transfer protein by apolipoproteins

Activation of human plasma lipid transfer protein (LTP) by apolipoproteins was studied. Pyrenelabeled cholesteryl ester was used as a probe substrate for the transfer reaction between lipid microemulsions, with a diameter of 26 nm, of triglyceride and phosphatidylcholine, and the reaction was monitored as a change in the ratio of the peaks of monomer and excimer in the fluorescence spectrum of pyrene. The transfer of pyrene-cholesteryl ester was hardly catalyzed by highly isolated LTP in the absence of apolipoprotein unless extreme overdose of LTP was given, regardless of the presence of bovine serum albumin. Human apolipoprotein (apo) A-I and apoA-II activated the LTP reaction in a dose-dependent manner. The activation was directly proportional to the titration of the surface of the substrate lipid emulsions by the apolipoproteins when the rate was plotted against the apolipoproteins bound to the surface. Human apoE also activated the LTP reaction in the same manner. The activation by human apoC-III was also proportional to the surface-bound protein, but the rate of the transfer was lower than those with other apolipoproteins. Displacement of apoA-I by apoC-III from the lipid emulsion surface, therefore, resulted in apparent deactivation of the LTP reaction. Thus, LTP requires apolipoproteins for its activation, and the activation seems proportional to the area of the surface of the lipid substrate particles modified by apolipoproteins. ApoA-I, -A-II, and -E are more potent activators than apoC-III for cholesteryl ester transfer.