Complete exchange of microsomal phospholipids by phospholipid-exchange protein does not alter the phospholipid asymmetry of these membranes

Abstract

surface pressure, and the pressure at which maximum condensation occurred depended on the structure of the phosphatidylcholine: below l.SpN/cm (15 dyn/cm) if no double bond is within 10 methylene units of the carboxyl end, or at about I.SpN/cm (15dyn/cm) if a double bond is present within 10 methylene units of the carboxyl end. The results were more involved for phosphatidylcholines containing C,, fatty acids, possibly as a result of the wide difference in chain length between the palmitic acid at the I-position and the C,, acid at the 2-position. Surface viscosity was measured by the oscillating-pendulum method (Gaines, 1966). A cylindrical pendulum suspended by a thin wire was caused to oscillate on either a clean water surface or on a lipid film maintained at a known surface pressure. The rate of damping of oscillations was measured. The pendulum bob was a cylinder of brass and Teflon, 3cm diameter, weight 238g and moment of inertia 658g.cm2. The tungsten suspension wire, approx. 0.012 cm in diameter, had a torsion constant of 13.1pN/cm (1 13dyn/cm). The pendulum assembly was lowered so that the Teflon base just made contact with the surface. The period of oscillation of this assembly was 14.1 s. Amplitudes of oscillation were measured by reflecting a light-beam (model 155 He-Ne laser, 0.95 mW, 623 nm; Spectra Physics, Mountain View, CA, U.S.A.) from a mirror on the suspension wire on to a graduated scale. The logarithms of the amplitudes plotted versus swing number produced straight or nearly straight lines, and from these plots the rate of decay of oscillation, Ks-' was obtained. On a clean water surface, the value of K was approx. 9 x 10-4s-1, and this value was subtracted from values of K obtained with monolayers to provide a measure of the surface viscosity of the monolayer (Evans et al., 1980). The viscosities of unsaturated phosphatidylcholines and sterols were undetectable with use of an oscillating pendulum, but the viscosities of saturated phospholipids were high and increased as surface pressure increased. All the sterols studied greatly decreased the viscosity of saturated phospholipids. The effect of cholesterol on the viscosity of dipalmitoylphosphatidylcholine was detected at a cholesterol concentration of 0.05 mo1/100mol, and at a cholesterol concentration of 4 mo1/100 mol the surface viscosity of dipalmitoylphosphatidylcholine was barely detectable. The results are interpreted as indicating that dipalmitoylphosphatidylcholine molecules exist in a monolayer as long linear polymers.