Cholesterol solubilization by short-chain lecithins: characterization of mixed micelles and cholesterol oxidase activity.

The synthetic short-chain lecithins diheptanoylphosphatidylcholine and dioctanoylphosphatidylcholine solubilize cholesterol up to 10 and 18 mol %, respectively. The half-time for diheptanoylphosphatidylcholine solubilization of solid cholesterol is 80 (+/- 30) min. This is much faster than Triton X-100 micelle or egg lecithin vesicle solubilization of solid cholesterol. Both the broadening of lecithin and [4-13C]cholesterol carbon resonances by Mn2+ and the observation of surface dilution kinetics for phospholipase A2 (Naja naja naja) and phospholipase C (Bacillus cereus) hydrolysis of the lecithins indicate that the cholesterol 3 beta-hydroxyl group resides at the particle surface exposed to solvent. Analysis of lecithin 13C chemical shifts suggests that cholesterol causes the short-chain lecithin acyl chains to become slightly more trans, although to a lesser extent than it affects egg lecithin chains in liposomes. Lecithin motion as characterized by 13C T1s and line widths is unaffected by the incorporation of cholesterol. [3,4-13C2]Cholesterol line widths are 5-10-fold narrower in these mixed micelles than in egg lecithin sonicated vesicles, while T1s in the two systems are comparable. These mixed micelles serve as substrates for cholesterol oxidase (Nocardia erythropolis) with a 40-fold rate increase over comparable cholesterol concentrations in egg lecithin vesicles. Part of this rate enhancement can be understood as an increase in interfacial area available to cholesterol oxidase in the micellar systems. These studies suggest that cholesterol oxidase has a weaker affinity for interfaces than other surface active enzymes.