Human apolipoprotein C-I accounts for the ability of plasma nigh density lipoproteins to inhibit the cholesteryl ester transfer protein activity

The aim of the present study was to identify the protein that accounts for the cholesteryl ester transfer protein (CETP)-inhibitory activity that is s pecifically associated with human plasma high density lipoproteins (HDL). T o this end, human HDL apolipoproteins were fractionated by preparative poly acrylamide gradient gel electrophoresis, and 30 distinct protein fractions with molecular masses ranging from 80 down to 2 kDa were tested for their a bility to inhibit CETP activity. One single apolipoprotein fraction was abl e to completely inhibit CETP activity. The N-terminal sequence of the 6-kDa protein inhibitor matched the N-terminal sequence of human apoC-I, the inh ibition was completely blocked by specific anti-apolipoprotein C-I antibodi es, and mass spectrometry analysis confirmed the identity of the isolated i nhibitor with full-length human apoC-I. Pure apoC-I was able to abolish CET P activity in a concentration-dependent manner and with a high efficiency ( IC50 = 100 nmol/liter). The inhibitory potency of total delipidated HDL apo lipoproteins completely disappeared after a treatment with anti-apolipoprot ein C-I antibodies, and the apoC-I deprivation of native plasma HDL by immu noaffinity chromatography produced a mean 43% rise in cholesteryl ester tra nsfer rates. The main localization of apoC-I in HDL and not in low density lipoprotein in normolipidemic plasma provides further support for the speci fic property of HDL in inhibiting CETP activity.