A meaty answer to a nosy question

Science News, Jan 10, 1998 by John Travis

Ah, the sweet smell of ... meat?

For one group of investigators, the odor of success is octanal, a molecule that most human noses perceive as a meaty smell. In the first case where a specific odor and its mammalian receptor have been definitively shown to work together, this team has identified a cell surface protein that enables rat nasal cells to perceive the octanal molecule.

Several years ago, scientists discovered a large family of genes, numbering as many as a thousand, all of which encode cell surface proteins made by the sensory nerve cells within the mammalian nose.

While investigators believe that these proteins act as receptors for odorants, the free-floating molecules sensed by the olfactory system, they have had trouble linking odorants to specific receptors.

To study a putative receptor called 17, a research group headed by Stuart Firestein of Columbia University engineered viruses to carry extra copies of the gene for 17 as well as a gene that encodes a fluorescent marker.

After infecting the rats' nasal cavities with the viruses, the scientists identified fluorescently labeled sensory cells and sprayed them with various odorants, one at a time. A device called an electro-olfactogram, which measures electric impulses generated within cells, enabled the researchers to determine whether the cells recognized any of 65 sprayed odorants.

For 64 of the odorants, the electro-olfactogram detected similar responses from both infected and uninfected nasal cells. For octanal, however, the response of infected cells was significantly quicker and stronger, presumably because the cells were binding the chemical with the additional copies of 17 on their surface.

The investigators then tested odorants that are structurally related to octanal. The infected cells responded to smells that had a meaty or waxy smell, but not to those that smelled more like grass or fruits, thus demonstrating the receptor's ability to distinguish small differences among odorants, Firestein and his colleagues report in the Jan. 9 SCIENCE.

By connecting specific odorants to receptors, researchers may learn which features of receptors are crucial to recognizing smells and how a thousand or so receptors can distinguish among the estimated 10,000 odorants, says Firestein.

"Every receptor is going to bind more than one thing, and every [odorant] is going to bind more than one receptor," notes olfactory researcher Glenn D. Prestwich of the University of Utah in Salt Lake City.