Modification of the N-terminal cysteine of plasma cholesteryl ester transfer protein selectively inhibits triglyceride transfer activity.

ABSTRACT

An invariant cysteine residue is found at the N-terminus of cholesteryl ester transfer protein (CETP) isolated from plasma of humans, rabbits and cynomolgus monkeys. We previously reported the expression of recombinant rabbit cholesteryl ester transfer protein in yeast (Kotake et al., J. Lipid Res. 1996; 37 599-605). The recombinant CETP secreted into the medium contains an altered N-terminal sequence but was fully capable of facilitating both cholesteryl ester (CE) and triglyceride (TG) transfer between lipoproteins. We investigated the importance of the conserved N-terminal cysteine of plasma CETP in the lipid transfer activity by chemical modification of the free sulfhydryl groups of the recombinant CETP and CETP from human and rabbit plasma. The unmodified forms of these CETPs had similar specific activities of CE and TG transfer. Neither 5,5'-dithiobis-(2-nitrobenzoate) nor N-ethyl maleimide altered the lipid transfer activity. In contrast, p-chloromercuriphenyl sulfonate selectively inhibited the TG transfer activity of both human and rabbit plasma CETP. The TG and CE transfer activities of the recombinant CETP, which lacks the N-terminal cysteine residue, was not affected. These results demonstrate that the N-terminal cysteine residue of both human and rabbit plasma CETP is free and is likely to be involved in the construction of a critical part of the active site of CETP that can determine the selectivity of the lipid molecule for the transfer reaction.