August 26, 2008 -- A team of electrochemists from the University of Warwick (Coventry, U.K.) used a confocal microscope and an ultramicroelectrode to study what really happens when a chemical crosses a cell membrane -- and were stunned by what they saw. Their observations suggest that key chemicals pass through cell walls up to 100 times more slowly than predicted by the century old "Overton's Rule." Because medicinal chemists have used this relationship to shape their studies and clinical trials for more than 100 years, the discovery could have major implications for the development and testing of future drugs.
Advances in technology enabled the researchers to position an ultramicroelectrode just 20 microns from the membrane boundary, and to generate a range of acids that should be able to diffuse relatively easily into a cell. These techniques allowed every step of the diffusion process to be directly examined. Previous studies had not been able to observe every step of the process and often required artificial stirring of the solutions.
Overton's rule says that the easier it is for a chemical to dissolve in a lipid (fat) the easier and faster it will be transported into a cell.
But the researchers observed that while the acids did diffuse across a lipid membrane, they did so at rates that were diametrically opposite to the predictions of the Rule, i.e. the most lipophilic molecules were actually transported slowest. The researchers studied four acids (acetic, butanoic, valeric, and hexanoic) that had increasingly larger "acyl" (or carbon) chains. The longer the carbon chain, the easier the chemical dissolves in lipids and, therefore, according to Ernst Overton of the University of Zürich, who outlined the rule in the 1890s, the faster they should diffuse across a lipid membrane. In fact, the University of Warwick researchers observed that for these four acids the exact opposite is true: the easier it is for an acid to dissolve in a lipid, the slower it is transported across the membrane.
The research team will now use their technique to examine the diffusion into cells of a range of other chemicals.
The research is published today in the Proceedings of the National Academy of Sciences (PNAS) and is entitled "Quantitative visualization of passive transport across bilayer lipid membranes".