The essential role of a free sulfhydryl group in blocking the cholesteryl site of cholesteryl ester transfer protein (CETP)

Cholesteryl ester transfer protein (CETP) has at least one unpaired sulfhydryl residue, which we have shown previously to be in or near the active site region. We investigated the location of this unpaired cysteine residue(s) of CETP using chemical modification with fluorescent sulfhydryl-specific reagents, limited proteolysis, and amino acid/sequence analysis. The kinetics of labeling CETP by either 2-(4'-maleimidylanilino)-naphthalene-6-sulfonic acid (MIANS) or acrylodan were followed by observing the increase in fluorescence of the bound probes. Labeling was inhibited strongly by preincubation of the CETP with either PNU-617, a competitive inhibitor of cholesteryl ester (CE) transport, and TP2 antibody. In addition, the transfer activities of the substrate CE by the modified CETP's were also inhibited but not competitively. Finally, preincubation of the native protein with N-ethylmaleimide (NEM) resulted in inhibition of activity that was dependent upon the time of exposure of the protein to the alkylating agent. These results provide further evidence that there is a cysteine residue in the active site region of CETP and ligands that either react or bind to this residue produce steric hindrance to CE transfer activity. Finally, although not conclusive, results of the protein chemistry experiments with the modified CETP suggest that the cysteine residue at position 333 is unpaired.