Zinc diffusion through lipid bilayers

Zinc diffusion across liposome bilayers was measured for a set of phosphatidylcholines. These lipids were sonicated to form small unilamellar vesicles in the presence of the metallochromic indicator antipyrylazo III. This chelator sequentially forms two complexes with zinc ion. The rate constant for the first complex formation is shown to increase linearly with zinc concentration. The slope of this line, a [Zn2+]-independent, second-order rate constant, varies with changes in phosphatidylcholine properties. The rate constant is little affected by changes in fluidity as estimated from the reduced temperature [Tr = (Texperimental-Tc)/Tc]. In contrast, the rate constant is directly dependent on lipid oxidation as measured by either a thiobarbituric acid test or a spectrophotometric determination of conjugate dienes. We estimate that zinc diffusion stimulated by lipid oxidation can approach rates observed in hepatocyte zinc transport.