Sweet and bitter: common origins?

Sweet and bitter: Common origins?

The tongue senses at least four discrete tastes: sweet, sour, salty and bitter. Scientists have assumed that taste buds discriminate among these using highly specific receptors, each sensing only one of the basic four. In fact, many studies have indicated that sweetness may involve multiple receptors that discriminate among various sugary flavors, notes Grant E. DuBois of NutraSweet Co. in Mt. Prospect, Ill. But his group's new research suggests that a single receptor responds to all sweet compounds -- and to bitter ones.

The NutraSweet researchers anesthetized rhesus monkeys and placed electrodes on a nerve behind the ear to "wiretap" electrical communications relayed to the brain from the tongue's sensory cells. To ensure that the wiretap was working and to establish a profile of typical responses, they presented the monkeys with each of the four tastes. Then they presented a solution containing either of two sweet-taste inhibitors. These newly developed inhibitors are chemical analogs of potent sweet compounds -- one based on a guanidine structure, the other on a structure of aryl urea.

Shining ultraviolet light on the monkeys' tongues after exposure to the guanidine-based inhibitor stimulated the formation of highly reactive compounds, which then permanently bonded to the nearest receptive chemical on the tongue. Because the inhibitor resembles a sweetener in chemical structure, the researchers expected it to bind to a sweetness receptor. And indeed, for several hours (the time it takes for a natural turnover in taste cells), any sweet compound passing over the tongue elicited only about half the original electrical response, DuBois reports.

Bitter compounds, however, triggered the same depressed response. This indicates the inhibitor had "deactivated" not only many of the sweet receptors but also those sensing bitterness, he says. The aryl-urea-based inhibitor similarly depressed both sweet and bitter recognition, though in this case the bond was temporary.

Within a few years, DuBois says, researchers should be able to radioactively tag the guanidine-based inhibitor to unmask the specific receptors responsive to sweet and bitter tastes, and then clone the receptors to identify their chemical structure.