Synthetic membranes smell and taste.

Synthetic membranes smell and taste

Thousands of molecules evoke smell and taste sensations in humans, but the mechanisms underlying their action remain unknown. Although many researchers suspect protein receptors in cell membranes within the taste buds and offactory tissue play key roles, scientists have not isolated such receptors, nor can they explain how a limited cast of receptors could each respond to thousands of structurally different bitter and odorous chemicals.

Yoshio Okahata and his colleagues at the Tokyo Institute of Technology  are testing an alternative hypothesis: that bitter and odorous substances trigger sensory cells in the tongue and nose by adhering to the cells' lipid membranes, and not by latching onto protein receptors. This adsorption presumably changes the electrical potential across the membranes, which in turn triggers specific neural-activity patterns corresponding to bitterness or specific smells.

To test this, the chemists made multilayer membranes out of synthetic lipids, measured how much of a particular bitter or odorous substance adsorbed onto their surfaces, and monitored changes in the membranes' electrical potential and resistance in response to the different chemicals.

"Such simple membrane systems may provide useful models of chemoreceptors in biological membranes," the scientists suggest in the July 15 ANALYTICAL CHEMISTRY. Sweet compounds, such as sucrose, and chemicals associated with other nonbitter tastes did not adsorb onto the membrane, they say, leaving open the possibility that receptors play a role in sensing them (SN: 5/19/90, p.315).

Indeed, the more intensely bitter or odorous a compound, the greater its tendency to adsorb onto the membrane. Strychnine and nearly all other bitter substances tested make the membrane's electrical potential more positive; odorous compounds such as camphor make it more negative. Bitter compounds have virtually no effect on a membrane's electrical resistance, whereas odorants decrease it. Many of these differences may reflect the adsorbed odorants' comparatively smaller and slinkier structures compared with bitter compounds, and their easier penetration into lipid membranes.