IN LEWIS CARROLL'S "Through the Looking Glass", Aliceinnocently wonders at one point whether looking-glass milk is good to drink.In fact, there is an answer to that question: looking-glass milk is bestavoided. The reason is that it would contain proteins that are mirror-imagesof the ones normally found in the body. And although molecules and theirmirror-images are chemically identical, our bodies are not used to mirror-imageproteins -- making them quite indigestible.

Indeed, though non-living matter generally contains equalnumbers of molecules and their mirror-images -- classified as either left-or right-handed, depending on their structure -- all living creatures (exceptbacteria) contain only the left-handed sort. Or so everyone thought, untila few years ago, when Japanese researchers found an amino acid called D-serinein mammalian brains. Unlike all other amino acids found in animals, D-serineis right-handed. Ever since, researchers have wondered why, and what itsfunction might be. In the current issue of the Proceedings of the NationalAcademy of Sciences, a team of researchers at Johns Hopkins Universitysuggests an answer.

The researchers have found an enzyme that converts serinefrom the normal left-handed form (called L-serine) to the exotic right-handedform. In this form, they suggest, D-serine acts as a super-selective keythat turns on a set of receptors in the brain called NMDA receptors, whichare involved in learning and memory and are stimulated by a neurotransmittercalled glutamate.

Too much NMDA activity, however, results in a stroke.And since the brain is literally flooded with glutamate, NMDA receptorsmust have some kind of security mechanism -- another, more selective chemicalwhich makes them receptive to glutamate. An amino acid called glycine hasbeen the prime candidate for some time. But Solomon Snyder, the head ofthe Hopkins group, now thinks that D-serine fits the bill better. Its uniqueright-handedness, he suggests, means that it is more selective than glycineabout whether or not it binds to NMDA, which makes it a superior securitymechanism.

Dr Snyder had his work cut out to prove this, since, unlikeother neurotransmitters, D-serine is not actually made in nerve cells.It is made in cells called glial cells, which are usually thought to functionmerely as insulation for more important nerve cells. But recently, hisgroup has shown that destroying D-serine in a culture of nerve cells stopsNMDA activity. And their new paper identifies the enzyme, called serineracemase, that converts left-handed serine into the right-handed form sothat it can do its job.

As well as solving the mystery of the right-handed molecule,this result could also have useful therapeutic consequences. By blockingthe enzyme that produces D-serine, it should be possible to suppress NMDAactivity, thus opening up the possibility of a new way to treat strokes.

Currently, most drugs try to block NMDA receptors themselvesin order to prevent overstimulation during a stroke. Now, says Dr Snyder,it will be possible to aim directly at the enzyme, by finding somethingthat stops it converting left-handed serine into D-serine. This secondapproach is easier, because drugs that aim at enzymes have tended to farebetter in clinical trials than those directed at receptors. Looking-glassmilk will never be good to drink; but looking-glass serine could proverather more useful.