Interactions of small molecules with phospholipid bilayers. Binding to egg phosphatidylcholine of some uncharged molecules (2-acetylaminofluorene, 4-dimethylaminoazobenzene, oestrone and testosterone) that bind to ligandin and aminoazo-dye-binding protein A

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Abstract
To assess the possible involvement of ligandin and aminoazo-dye-binding protein A in intracellular transport, how their ligands, most of which are molecules with hydrophobic moieties, interact with cellular membranes must be known. To obtain such information the interactions of 2-acetylaminofluorene, 4-dimethylaminoazobenzene, estrone and testosterone with aqueous dispersions of egg phosphatidylcholine and egg phosphatidylcholine/cholesterol (1:1, molar ratio ) were studied by equilibrium dialysis and spectrophotometry. At 25.degree. C and pH 7.4, the partition coefficients for binding to phosphatidylcholine [expressed as (mol of ligand bound/mol of phosphatidylcholine)/unbound ligand concentration] were: 2-acetylaminofluorene, 5.0 .times. 103 l/mol; 4-dimethylaminoazobenzene, 2.1 .times. 104 l/mol; estrone, 3.1 .times. 103 l/mol; testosterone, 4.2 .times. 102 l/mol. In the ranges studied these values were independent of concentration. The results for the 2 steroids confirm those of Heap et al. The introduction of cholesterol into the lipid bilayers caused large decreases in the partition coefficients of estrone and testosterone but had relatively little effect on the binding of 2-acetylaminofluorene and 4-dimethylaminoazobenzene. By assuming that the interactions with egg phosphatidylcholine bilayers resemble those with the phospholipid components of mammalian intracellar membranes, the phosphatidylcholine partition coefficients and data for binding to the intracellular proteins ligandin and aminoazo-dye-binding protein A enable the subcellular distributions of the 4 compounds to be estimated. For the rat hepatocyte up to 98, 99, 89 and 58% of the total 2-acetylaminofluorene, 4-dimethylaminoazobenzene, estrone and testosterone, respectively, may be membrane-bound.

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