Determinants of Xenobiotic Transport at Biological Barriers

Jackson, Michael; Cohn, Victor

doi:10.1002/cphy.cp090125

Cited by 3 publications

(2 citation statements)

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“…They suggest that this displacement is due to a negative charge on the membrane or in the cell wall, which causes a decrease in the pH near the membrane surface and a consequent increase in the apparent pK of the weak acid. Similar arguments have been made by Bean et al (1968) and Jackson and Cohn (1977). These arguments are incorrect, however, because the negative surface charge causes equal and opposite effects on the aqueous concentrations of H + and A-.…”

Section: Auxin Permeability Of Lipid Bilayers and Plant Cell Membranesmentioning

confidence: 78%

Transport of auxin (indoleacetic acid) through lipid bilayer membranes

Gutknecht

Walter

1980

J. Membrain Biol.

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Summary. Diffusion of auxin (indole-3-acetic acid) through planar lipid bilayer membranes was studied as a function of pH and auxin concentration. Membranes were made of egg or soybean lecithin or phosphatidyl serine in n-decane (25-35 mg/ml). Tracer and electrical techniques were used to estimate the permeabilities to nonionized (HA) and ionized (A) auxin. The auxin tracer flux is unstirred layer limited at low pH and membrane limited at high pH, i.e., when [A-] >> [HA]. The tracer flux is not affected by the transmembrane voltage and is much higher than the flux predicted from the membrane conductance. Thus, only nonionized auxin crosses the membrane at a significant rate. Auxin transport shows saturation kinetics, but this is due entirely to unstirred layer effects rather than to the existence of an auxin "carrier" in the membrane. A rapid interconversion of A-and HA at the membrane surface allows A-to "facilitate" the auxin flux through the unstirred layer. Thus, the total flux is higher than that expected for the simple diffusion of HA alone. . By fitting this equation to our data we find that Per =6.9 x 10 -4 cm/sec and Pn~ = 3.3 x 10 -3 cm/sec for egg lecithin-decane bilayers. Similar membrane permeabilities were observed with phosphatidyl serine or soybean lipids. Thus, auxin permeability is not affected by a net surface charge on the membrane. Our model describing diffusion and reaction in the unstirred layers can explain the "anomolous" relationship between pH and weak acid (or weak base) uptake observed in many plant cells.The transport of auxin (indole-3-acetic acid) plays a major regulatory role in the growth and development of plants (Goldsmith, 1977). Closely related indole

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