Reaction of human lecithin: cholesterol acyltransferase with micellar substrates is independent of the phase state of the lipid.

Micellar complexes with different phosphatidylcholine (PC) compositions were prepared by the dialysis of PC-cholesterol dispersions with cholate in the presence of human apolipoprotein A-I (apo A-I). The complexes isolated by gel filtration had molecular weights around 200 000, two apo A-I molecules per particle, PC to apo A-I molar ratios from 91 to 123, and cholesterol to apo A-I molar ratios from 6 to 11. The phase-transition behavior of these complexes was examined by fluorescence polarization of diphenylhexatriene: the complexes containing dimyristoyl-PC had a transition temperature (Tm) of 32 degrees C, the complexes with dipalmitoyl-PC had a Tm of 45 degrees C, and those prepared with palmitoyloleoyl-PC were mostly present in the liquid-crystalline state in the temperature range investigated (55-7 degrees C). The initial velocities of the enzymatic reaction with purified human lecithin: cholesterol acyltransferase decreased in the order palmitoyloleoyl-PC greater than dipalmitoyl-PC greater than dimyristoyl-PC, at saturating micellar substrate levels. Arrhenius plots of the reaction rates from 15 to 41 degrees C were linear, and the activation energies ranged from 20 to 30 kcal/mol. These results indicate a marked dependence of the enzymatic reaction rates on the nature of the acyl donor, a dependence which is not related to the phase state of the bulk lipid in the micellar complexes.