Commercial cholesterol oxidase from R. erythropolis was purchased from Boehringer Mannheim (25 U/mg). Cholesterol from lanolin, cholesterol from human gallstones, Triton X-100 and Triton X-114 were purchased from Fluka. Triton X-114 was condensed three times in 5 mM sodium phosphate buffer pH 7.5 [30,31]. The detergent phase of the third condensation had a concentration of 25% w/v TX-114 and was used as the detergent stock solution for all the experiments. The Triton X-114 concentration was determined from its absorption at 278 nm (A278 = 1.25 for 0.05% w/v) [29]