A team at Hebrew University in Jerusalem claim that certain bitter and artificial sweet taste molecules (or artificial sweeteners) somehow enter the taste-bud cells, where they inhibit the natural termination of the taste-receptor signal, resulting in what we call 'aftertaste'.
"By inhibiting the phosphorylation of the taste sensors, the receptors continue to be active, and so we continue to taste what is often an unwelcome sensation to begin with," says lead researcher Naim.
There may be more than one mechanism at work and theoretically there are other possible approaches to this complex phenomenon, but so far this hypothesis has held up to experimentation, he adds.
How consumers taste food is crucial knowledge for a food industry constantly organising the building blocks of new food formulations.
In addition, rising health concerns in society is seeing consumers turning towards sugar-free products, pushing food makers to introduce zero-calorie or low-calorie sugar substitutes into their new product formulations.
And this leap in demand is the dynamo behind strong growth for the market, pitched to rise about 8.3 per cent year on year until 2008, far outpacing industry growth currently at around 3 to 4 per cent.
But the aftertaste, often bitter, that artificial non-sugar sweeteners is a challenge for food developers.
In recent years, researchers have identified receptors for sweet and bitter tastes. These receptors belong to the family of G protein coupled receptors (GPCRs) and are found on the plasma membrane of taste cells.
In general, stimulation of this type of receptor leads to intracellular formation of such second messengers as IP3, cAMP, cGMP, as well as activation of some ionic channels.
"Termination of this signalling in most cases is initiated by receptor phosphorylation, a kind of common physiological 'on/off switch,'" explains Naim.
Findings reported in the August issue of the American Journal of Physiology-Cell Physiology," showed that GRK5 and perhaps GRK2 and GRK6 are present in taste-bud cells.
"Furthermore, we show that the phosphorylation of rhodopsin, which we used as a model for GPCR, by GRK5, GRK2 was inhibited in vitro by a variety of non-sugar sweeteners and bitter tastants," says Naim.
The tastants included the artificial sweeteners saccharin, NHD, cyclamate, D-tryptophan and acesulfame K, and (in the bitter spectrum) cyclo(Leu-Trp), caffeine, quinine, L-tryptophan, limonin and naringin.
The phosphoryalization activity of protein kinase A (PKA), another receptor-related kinase, was also inhibited by these tastants, report the scientists.
Naim warned that much still remains to be proven.
"Additional studies using the newly discovered taste GPCRs are needed to show their interaction with GRKs and possibly with other kinases, such as in intact cells in vivo, before anything can be unequivocally stated," he adds.
Because these tastants are components of our daily diets and may access other tissues along the gastrointestinal tract, these results may have implications for cellular signalling in tissues other than those involved in taste, conclude the scientists.