The aim of the present study was to determine in vitro the effects of various purified apolipoproteins (apo) on the activity of the cholesteryl ester transfer protein (CETP). It appeared that the ability of apoA-I, A-II, and A-IV to modulate the CETP-mediated transfer of radiolabeled cholesteryl esters between low density lipoproteins (LDL) and high density lipoproteins (HDL) was markedly influenced by the final apolipoprotein:lipoprotein ratio in incubation mixtures. At low apolipoprotein:lipoprotein ratio, the rate of radiolabeled cholesteryl esters transferred from HDL3 to LDL was significantly increased in the presence of apoA-I and apoA-IV. Under similar conditions, the rate of radiolabeled cholesteryl esters transferred from LDL to HDL3 was increased in the presence of apoA-I while apoA-IV had no significant effects. At high apolipoprotein:lipoprotein ratio, the ability of apoA-I and apoA-IV to enhance the rate of radiolabeled cholesteryl esters transferred either from HDL3 to LDL or from LDL to HDL3 was considerably reduced. At the highest apolipoprotein:lipoprotein ratio studied, apoA-I and A-IV became inhibitors of the CETP-mediated transfer reaction. Interestingly, apoA-II differed markedly from other apolipoproteins as, even at a low apolipoprotein:lipoprotein ratio, it significantly inhibited CETP activity as measured either from HDL3 to LDL or from LDL to HDL3. The inhibition by apoA-II was concentration-dependent and, at the highest apolipoprotein:lipoprotein ratio studied, cholesteryl ester transfer activity was totally suppressed. The possibility of a direct interaction between CETP and the two major HDL apolipoproteins, apoA-I and apoA-II, was further investigated by combining crosslinking and immunoblotting techniques. Whereas CETP alone had an apparent molecular mass of 76,000 +/- 3,100 Da, crosslinking reactions in incubation mixtures containing CETP and either apoA-I or apoA-II revealed the appearance of additional protein bands with apparent molecular masses of 99,600 +/- 6,100 and 86,900 +/- 4,500 Da, respectively. These complexes corresponded to the association of one molecule of CETP with one molecule of apoA-I or apoA-II. Interestingly, the mass concentrations of apoA-II needed to produce visible CETP-apolipoprotein complexes appeared to be about ten times higher as compared with apoA-I, suggesting that CETP may have a lower affinity for apoA-II than for apoA-I. In conclusion, data from the present study indicate that apolipoproteins A-I, A-II, and A-IV could be potent modulators of the CETP-mediated transfer of cholesteryl esters between HDL and LDL fractions.(ABSTRACT TRUNCATED AT 400 WORDS)