Monitoring the Anisotropy and Fluidity of the HDL Monolayer as Surrogates of HDL Functionality.

The fluidity of the biological lipid layers modulates processes involved in cardiovascular disease. High-density lipoprotein (HDL) monolayer fluidity is considered as a surrogate of HDL functionality. In particular, the more fluid the HDL monolayer is, the greater the cholesterol efflux (ChE) is observed. Fluidity depends on cholesterol and on the saturation and length of the fatty acids present in lipid layers. Specifically, low cholesterol and short-chain and/or low-saturated fatty acids content in the lipid layers increases fluidity. Lipid peroxidation is also involved in regulating the monolayers' fluidity. HDL oxidation decreases its fluidity and ChE capacity. Accordingly, the presence of antioxidants in biological membranes and in HDL increases fluidity. The fluidity is assessed in polarization studies that measures the steady-state anisotropy (r) using fluorescent probes (such as 1,6-diphenyl-1,3,5-hexatriene; DPH) that mimic the molecular movements of the sample analyzed. Since r refers to the rigidity and fluidity refers to the viscosity of lipid layers, the fluidity index is the inverse value of r (i.e., 1/r). This chapter describes a method for measuring HDL monolayer fluidity and r. The reproducibility of this method was excellent as the intra-assay coefficients of variation (CV) were <2.5 (20 replicates on the same day) and the interassay CV were <5% (60 replicates measured on 3 different days; 20 replicates/day). The method therefore represents a reproducible and useful tool to evaluate HDL functionality as an emerging cardiovascular risk factor.