THE INFLUENCE OF SPHINGOMYELIN ON THE STRUCTURE AND FUNCTION OF RECONSTITUTED HIGH-DENSITY-LIPOPROTEINS

The effect of sphingomyelin (SPM) on the structure and function of dis coidal and spherical reconstituted high density lipoproteins (rHDL) ha s been studied. Three preparations of discoidal rHDL with 1-palmitoyl- 2-oleoyl phosphatidylcholine (POPC)/SPM/unesterified cholesterol (UC)/ apolipoprotein (apo)A-I molar ratios of 99.6/0.0/10.2/1.0, 86.0/13.6/1 0.8/1.0, and 72.5/26.3/11.4/1.0 were prepared by cholate dialysis. SPM did not affect discoidal rHDL size or surface charge. Esterification of cholesterol by lecithin:cholesterol acyltransferase (LCAT) was inhi bited in the SPM-containing discoidal rHDL, When the discoidal rHDL of POPC/SPM/UC/ apoA-I molar ratio 99.6/0.0/10.2/1.0 were incubated with low density lipoproteins (LDL) and LCAT, SPM transferred spontaneousl y from the LDL to the rHDL (t(1/2) = 0.8 h) and spherical particles wi th a POPC/SPR4/UC/CE/ apoA-I molar ratio of 24.6/4.9/3.6/24.9/1.0 were formed. Depleting the spherical rHDL of SPM head groups by incubation with sphingomyelinase increased the negative charge on the surface, b ut did not change their size, Cholesteryl ester transfer protein (CETP )-mediated transfers of cholesteryl esters and triglyceride between sp herical rHDL and Intralipid were not affected by SPM head group deplet ion, The effect of SPM on rHDL structure was assessed spectroscopicall y. SPM increased POPC acyl chain and head group packing in the discoid al rHDL. When the spherical rHDL were depleted of SPM head groups, POP C acyl chain packing order decreased, but head group packing order was not affected. SPM inhibited the lipid-water interfacial hydration of discoidal rHDL, This parameter was not affected when the spherical rHD L were depleted of SPM head groups. The SPM molecule and the SPM head group, respectively, inhibited the unfolding of apoA-I in discoidal an d spherical rHDL. It is concluded that (i) SPM influences the structur e of discoidal and spherical rHDL, (ii) SPM inhibits the LCAT reaction in discoidal rHDL, and (iii) the SPM head group does not affect CETP- mediated lipid transfers into or out of spherical rHDL.