Overturning Overton: a venerable rule goes down [Updated]

You might think it is an arcane subject, but a paper just published in the Proceedings of the National Academy of Sciences (PNAS), "Quantitative visualization of passive transport across bilayer lipid membranes" by Grime et al., is quite a stunner. This paper is about a century old formula called the Overton Rule (or Meyer - Overton Rule), used extensively to predict how fast chemicals get into cells and which ones do so most easily. It has been used to predict which anesthetics would work best and how fast and which toxic chemicals would get into which cells and how fast. For those of us who deal with the health effects of chemicals, it is a basic assumption that fat soluble chemicals get into cells more easily than water soluble ones (on average).

A cell membrane (the cell's outer boundary) is a lipid bilayer, i.e., two opposed layers (hence bilayer). Each layer has two sides, one water soluble and the other fat soluble. The two layers are lined up so the two fat soluble sides face each other, like a fat sandwich with water soluble bread on each side. The water soluble surfaces face the interior of the cell (which is mainly water) and the exterior of the cell (also mainly water). Overton's Rule was a quantitative description of how fat solubility of the chemical affected the ease with which a substance diffused from outside the cell to the inside. The more fat soluble, the more easily the chemical would pass into the cell. Or so we thought:

A team of electrochemists from the University of Warwick used a combination of a confocal microscope and an ultramicroelectrode to study what really happens when a chemical crosses a cell membrane. Advances in technology enabled them to position an ultramicroelectrode incrediblely close to the membrane boundary (roughly 20 microns) where it was used 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.

The results stunned the researchers. 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 [fat loving] 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 Overton, 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. (Press Release, U. of Warwick)

There has been a lot of work trying to figure out the basic biophysics supporting the Overton Rule, i.e., what mechanisms going on at the molecular and thermodynamic level would give rise to the Overton relationship. It now appears all that work, along with a basic Rule of Thumb used by toxicologists, pharmacologists and anesthesiologists may have to be rethought.

You might think this would be utterly dismaying for scientists. Just the opposite. This is the kind of stuff we live for.

Update: In the comments, sciblings Coturnix (A Blog Around the Clock) and Orac (Respectful Insolence) point us to an extremely interesting response to the U. of Warwick work by pharmacologist scibling Abel at Terra Sigallata registering some skepticism about the novelty and importance of this work. It appears that neither of us were able to see the publication (press releases about papers that are still inaccessible to the scientific community are becoming more common and PNAS is one of the prime offenders in my experience), but Abel as a pharmacologist has more experience and perspective on this rule of pharmacology and I bow to his expertise. I regret I missed his earlier post.

My reaction was based on two things. The first is that the idea behind the Meyer-Overton Rule comes from reading the risk assessment and toxicology literature where the idea that lipophilicity and easy passage through the membrane are related is a rule of thumb and this was a useful caution about such a too facile interpretation. The second has to do with the quantitative relations implied by the rule and its relation to the partition coefficient. A scan at the literature prior to writing the post suggested that biophysicists have been struggling to "explain" the rule quantitatively by various models that took thermodynamical effects into account and that a revision in the scope and veracity of the rule might well affect our notions about what is going on as molecules encounter and pass through the cell membrane.

In any event, please read Abel's superb post on this.