Lipid Domains in Plasma Membrances from Rat Liver

Abstract

The existence of fluid and solid lipid domains in isolated rat‐liver plasma membranes was evaluated using the fluorescent fatty acids trans‐parinaric and cis‐parinaric acid as probe molecules for solid and fluid membrane areas, respectively. The fluorescence probe 1,6‐diphenyl‐1,3,5‐hexatriene indicated that a phase transition was present in the liver plasma membrane between 18°C and 30°C. At intermediate temperatures, cis‐parinaric acid, which partitioned approximately equally into fluid and solid lipid areas, detected two lipid domains: the mole fractions of fluid and solid lipid domains at 24°C were 0.32 and 0.68 while the mole fractions of cis‐parinaric acid in each domain were 0.34 and 0.66, respectively. The dissociation constant, aqueous to membrane lipid partition coefficient, and bound to free ratio for trans‐parinaric acid were 7.0 ± 0.7 μM, 4.0 ± 0.6 × 10⁶, and 83:17, respectively. The affinity of the membrane for cis‐parinaric acid was twofold lower than for trans‐parinaric acid. The trans‐parinaric acid partitioned preferentially into solid lipid. K_p^s/f= 3.30, while the cis‐parinaric acid partitioned equally between fluid and solid phases K_p^s/f= 0.92. Thus, the data demonstrate the coexistence of fluid and solid domains in rat liver plasma membranes.