EVIDENCE THAT CYNOMOLGUS MONKEY CHOLESTERYL ESTER TRANSFER PROTEIN HAS 2 NEUTRAL LIPID-BINDING SITES

Two inhibitors of cynomolgus monkey cholesteryl ester transfer protein were evaluated. One, a monoclonal antibody made against purified cyno molgus monkey cholesteryl ester transfer protein, was capable of sever ely inhibiting triglyceride transfer, but had a variable effect on cho lesteryl ester transfer. At low antibody to antigen ratios, there was what appeared to be a stoichiometric inhibition of cholesteryl ester t ransfer, but at high antibody to antigen ratios the inhibition of chol esteryl ester transfer was completely relieved, even though triglyceri de transfer remained blocked. Fab fragments of the antibody had no eff ect whatsoever on cholesteryl ester transfer, but were capable of comp letely blocking triglyceride transfer. The other inhibitor, 6-chlorome curic cholesterol, severely inhibited cholesteryl ester transfer with minimal inhibition of triglyceride transfer. When both inhibitors were added to the assay, both cholesteryl ester and triglyceride transfer were inhibited; an indication that the inhibitors did not compete for the same binding site on cholesteryl ester transfer protein. When the antibody was given subcutaneously to cynomolgus monkeys at a dose whic h inhibited triglyceride transfer in the plasma by more than 90%, ther e was no detectable effect on the high density lipoprotein (HDL) chole sterol level, but the HDL triglyceride levels decreased from 13 +/- 2 to 1 +/- 0 mol/mol of HDL (mean +/- S.D.); an indication that the anti body uncoupled cholesteryl ester and triglyceride transfer in vivo. Th e 6-chloromecuric cholesterol could not be evaluated in vivo because i t is a potent lecithin:cholesterol acyltransferase inhibitor. The fact that cholesteryl ester transfer can be inhibited without effect on tr iglyceride transfer and, conversely, that triglyceride transfer can be inhibited without effect on cholesteryl ester transfer indicates that these two lipids are not transferred by a single, non-discriminatory process.