Modulation of the kinetics of 3β-hydroxy-5-oxo-5,6-secocholestan-6-al/phosphatidylethanolamine Schiff base formation by cholesterol and cholesterol crystallization

We have previously shown that the oxidized cholesterol 3β-hydroxy-5-oxo-5,6-secocholestan-6-al (atheronal A) reacts covalently with the free amino group of phosphatidylethanolamine (PE) or phosphatidylserine (PS) to produce a Schiff base. Accompanying this interaction, the biophysical properties of the phospholipid membranes are also changed. In the present report, we extend our earlier study of the rate of Schiff base formation in dimyristoyl PE/atheronal A binary mixtures to the more biologically relevant case in which varying amounts of cholesterol are also present. Using optical spectroscopy to monitor reaction kinetics, we demonstrate that the presence of cholesterol reduces the accessibility of the aldehyde moiety of the atheronal A to the free headgroup amine. We also find that the presence of atheronal A promotes the early onset of cholesterol crystallization in the ternary mixtures, perhaps with the Schiff base serving as a site for heterogeneous nucleation.