Distribution of hydrophobic ionizable xenobiotics between water and lipid membranes: pentachlorophenol and pentachlorophenate. A comparison with octanol-water partition.
We have studied distribution of pentachlorophenol (PCP)--a major environmental pollutant--between egg-phosphatidylcholine (egg-PC) membranes and water. The objectives were (1) to compare the membrane-water partition of the un-ionized (HA) and ionized (A) PCP, and (2) to establish similarities and differences between the partition of PCP into lipid membranes and into octanol. The studies were made with egg-PC liposomes. It is shown that the distribution isotherms can be understood in terms of the Langmuir-Stern-Grahame adsorption model. The model is applicable to both the HA and A species; it takes into account the electrostatic interactions at the membrane-water interface charged by the adsorbed pentachlorophenate. Relationships between the membrane surface adsorption and bulk partition characteristics were presented and used to relate the partition of PCP into egg-PC membranes to those for octanol-water systems. Results (egg-PC membranes): bulk distribution coeff. gamma HA = 2.9 x 10(5), gamma A = 1.6 x 10(4), association constant KmHA = 2.9 x 10(5) M-1, KmA = 0.7 x 10(5) M-1, adsorption site area PsHA = 0.6 nm2, PsA = 3.5 nm2, and linear partition coeff. Beta mHA = 550 microns, beta mA = 30 microns. Comparable to gamma HA and gamma A for octanol-water are P(ow)(HA) approximately 1.3 x 10(5) and P(ow)(A) approximately 30. The major difference is in the distribution of ionized PCP which is several hundred times greater for egg-PC membranes compared to octanol. The difference is associated with the properties of the membrane-water interface.
Smejtek
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Department of Physics, Portland State University, Oregon 97207-0715.
Wang
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D008567:Membranes, Artificial; D000442:Octanols / Q000737:chemistry; D010416:Pentachlorophenol / Q000032:analysis*; D010713:Phosphatidylcholines / Q000737:chemistry*; D012995:Solubility; D014867:Water / Q000737:chemistry; D014874:Water Pollutants, Chemical / Q000032:analysis*; D015262:Xenobiotics / Q000032:analysis*
D008567:Membranes, Artificial; D000442:Octanols; D010713:Phosphatidylcholines; D014874:Water Pollutants, Chemical; D015262:Xenobiotics; D014867:Water; D010416:Pentachlorophenol