CHARACTERIZATION OF CHOLESTERYL ESTER TRANSFER PROTEIN INHIBITOR FROMPLASMA OF BABOONS (PAPIO SP)

Selective breeding has produced families of baboons that accumulate la rge high density lipoproteins (HDL,) when challenged with a high chole sterol and high fat (HCHF) diet. In the plasma isolated from these hig h HDL1 baboons there is a factor that decreases the transfer of choles teryl ester from HDL to lower density lipoproteins. The purpose of the se studies was to identify and characterize this inhibitor of choleste ryl ester transfer. A protein with molecular mass of approximately 4 k Da 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 ma ss 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 a poC-I, the fragment is also a slightly basic protein (pI 7.1). The apo C-I fragment and modified apoA-I presented at equimolar concentrations exhibited similar inhibition of cholesteryl ester transfer protein (C ETP) activity in HDL of low HDL, 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 wa s synthesized chemically. This synthetic peptide also inhibited CETP a ctivity in vitro. Rabbit polyclonal antisera prepared against the 38 a mino acid synthetic peptide recognized the 4 kDa molecular mass inhibi tor 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 t he other hand, the antibody detected only apoC-I in the plasma lipopro teins of low HDL1 baboons. The IgG fraction isolated from antiserum ra ised against the synthetic inhibitor peptide increased cholesteryl est er transfer from HDL of high HDL1 baboons, whereas the IgG antibody ag ainst CETP decreased cholesteryl ester transfer from HDL of both high and low HDL, baboons. These studies suggest that the CETP inhibitor is an N-terminal fragment of apoC-I, and this fragment also modifies apo A-I and apoE in the plasma.