Distribution of cholesterol between the outer and inner halves of the lipid bilayer of mycoplasma cell membranes

Rapid kinetic studies of filipin binding to intact cells and isolated membranes were performed with a stopped-flow apparatus to determine the distribution of cholesterol in the outer and inner surfaces of mycoplasma membranes. The initial rates of association of filipin with cholesterol in Mycoplasma gallisepticum and Mycoplasma capricolum intact cells were slower than those obtained with isolated membrane preparations. Ratios of the second-order rate constants for filipin binding to cells relative to membranes indicate that cholesterol is distributed symmetrically in membranes of M. gallisepticum cells whereas in M. capricolum ～66% of the free cholesterol is localized in the outer half of the lipid bilayer.