Characterization of cholesteryl ester transfer protein inhibitor from plasma of baboons (Papio sp.)

Selective breeding has produced families of baboons that accumulate large high density lipoproteins (HDL1) when challenged with a high cholesterol and high fat (HCHF) diet. In the plasma isolated from these high HDL1 baboons there is a factor that decreases the transfer of cholesteryl ester from HDL to lower density lipoproteins. The purpose of these studies was to identify and characterize this inhibitor of cholesteryl ester transfer. A protein with molecular mass of approximately 4 kDa was detected in greater amounts in the plasma lipoproteins of high HDL1 baboons fed the HCHF diet than in plasma lipoproteins of low HDL1 baboons. This 4 kDa protein appeared to associate with apolipoprotein (apo) A-I, resulting in modified apoA-I with an apparent molecular mass of 31 kDa. A small amount of modified apoE was also identified with a molecular mass of 41 kDa. N-terminal amino acid sequencing of the 4 kDa peptide identified it as an N-terminal fragment of apoC-I. Like apoC-I, the fragment is also a slightly basic protein (pI 7.1). The apoC-I fragment and modified apoA-I presented at equimolar concentrations exhibited similar inhibition of cholesteryl ester transfer protein (CETP) activity in HDL of low HDL1 baboons. On the basis of baboon apoC-I amino acid sequence and the molecular mass of the inhibitor peptide, a peptide corresponding to the N-terminal 38 amino acids of apoC-I was synthesized chemically. This synthetic peptide also inhibited CETP activity in vitro. Rabbit polyclonal antisera prepared against the 38 amino acid synthetic peptide recognized the 4 kDa molecular mass inhibitor protein, apoC-I (6.6 kDa), and the modified apoA-I protein (31 kDa molecular mass) in the plasma lipoproteins of high HDL1 baboons. On the other hand, the antibody detected only apoC-I in the plasma lipoproteins of low HDL1 baboons. The IgG fraction isolated from antiserum raised against the synthetic inhibitor peptide increased cholesteryl ester transfer from HDL of high HDL1 baboons, whereas the IgG antibody against CETP decreased cholesteryl ester transfer from HDL of both high and low HDL1 baboons. These studies suggest that the CETP inhibitor is an N-terminal fragment of apoC-I, and this fragment also modifies apoA-I and apoE in the plasma.