Effects of sterol structure and exogenous lipids on the transbilayer distribution of sterols in the membrane of Mycoplasma capricolum

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Abstract
Stopped-flow kinetic measurements of the association of filipin with sterols in intact cells and isolated membranes of M. capricolum were used to study the effects of varying the phospholipid composition and the sterol structure on sterol distribution in the membrane. The phospholipid composition and content of the membrane were varied by growing cells in an albumin-containing medium with cholesterol, palmitic and oleic acids, and various concentrations of exogenous phospholipids. The exogenous phospholipids (phosphatidylcholine, sphingomyelin and phosphatidic acid) were incorporated up to levels of .apprx. 50% of the total membrane phospholipids but had no effect on the distribution of cholesterol between the 2 halves of the membrane bilayer. The sterol structure was varied by growing the cells with 10 .mu.g/ml of either cholesterol, .beta.-cholestanol, 4,6-cholestadien-3.beta.-ol, ergosterol, .beta.-sitosterol or stigmasterol. With cholesterol, .beta.-cholestanol and 4,6-cholestadien-3.beta.-ol, .apprx. 65% of the sterol was present in the outer half of the lipid bilayer. With ergosterol, .beta.-sitosterol and stigmasterol, .apprx. 89% of the sterol is localized in the outer half of the membrane bilayer. Thus, the behavior of the alkyl-substituted sterols differs from that of cholesterol. The extent to which a sterol is distributed asymmetrically between the 2 halves of the bilayer is not related to the extent to which maximum growth is produced. Growth-supporting sterols apparently need not be translocated extensively.