Hydrolysis of cholesteryl oleate liquid crystals by hormone-sensitive lipase

Cholesteryl oleate liquid crystals were prepared by sonication as a model of lipid droplets accumulated in foam cells derived from macrophages. These liquid crystals were spherical and showed an anisotropic cross image. Hormone-sensitive lipase from bovine adipose tissue hydrolyzed these liquid crystals optimally at pH 6.8. The Km for the liquid crystals was about 8 times that for vesicles or emulsified cholesteryl oleate. The Vmax for the liquid crystals was the same as that for the emulsified substrate and 6 times that for the vesicle substrate. Phospholipid inhibited cholesteryl oleate hydrolysis in a concentration-dependent fashion, phosphatidylserine being especially inhibitory. The effect of phospholipids on the activity changed upon their incorporation into the cholesteryl oleate liquid crystals, phosphatidylethanolamine increasing the activity to about twice that of the control. These results suggest that hormone-sensitive lipase hydrolyzes cholesteryl oleate liquid crystals as a model of endogenous lipid droplets and its activities are affected by phospholipids.