Phosphatidylcholine fluidity and structure affect lecithin : cholesterol acyltransferase activity

The purpose of this study was to test the hypothesis that lipid fluidity re gulates lecithin:cholesterol acyltransferase (LCAT) activity. Phosphatidylc holine (PC) species were synthesized that varied in fluidity by changing th e number, type (cis vs. trans), or position of the double bonds in 18 or 20 carbon sn-2 fatty acyl chains and recombined with [H-3]cholesterol and apo lipoprotein A-I to form recombinant high density lipoprotein (rHDL) substra te particles, The activity of purified human plasma LCAT decreased with PC sn-2 fatty acyl chains containing trans versus cis double bonds and as doub le bonds were moved towards the methyl terminus of the sn-2 fatty acyl chai n, The decrease in LCAT activity was significantly correlated with a decrea se in rHDL fluidity (measured by diphenylhexatriene fluorescence polarizati on) for PC species containing 18 carbon (r(2) = 0.61, n = 18) and 20 carbon (r(2) = 0.93, n = 5) sn-2 fatty acyl chains, rHDL were also made containin g 10% of the 18 carbon sn-2 fatty acyl chain PC species and 90% Of an inert PC ether matrix (sn-1 18:1, sn-2 16:0 PC ether) to normalize rHDL fluidity . Even though fluidity was similar among the PC ether-containing rHDL, the order of PC reactivity with LCAT was significantly correlated (r(2) = 0.71) With that of 100% PC rHDL containing the same 18 carbon sn-2 fatty acyl ch ain species, suggesting that PC structure in the active site of LCAT determ ines reactivity in the absence of measurable differences in bilayer fluidit y We conclude that PC fluidity and structure are major regulators of LCAT a ctivity when fatty acyl chain length is constant.