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

Abstract

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 MW around 200,000, 2 apo A-I molecules/particle, PC to apo A-I molar ratios from 91-123, and cholesterol to apo A-I molar ratios from 6-11. 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.degree. C, the complexes with dipalmitoyl-PC had a Tm of 45.degree. C, and those prepared with palmitoyloleoyl-PC were mostly present in the liquid-crystalline state in the temperature range investigated (55-7.degree. C). The initial velocities of the enzymatic reaction with purified human lecithin:cholesterol acyltransferase decreased in the order palmitoyloleoyl-PC > dipalmitoyl-PC > dimyristoyl-PC, at saturating micellar substrate levels. Arrhenius plots of the reaction rates from 15-41.degree. C were linear, and the activation energies ranged from 20-30 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.