Biochemistry of sweet taste transduction

Abstract

Recent biochemical and biophysical studies point to mechanistic hypotheses for sweet taste transduction involving either second messengers or stimulus-gated ion channels. Biochemical studies have shown that sweet tasting stimuli enhance the production of the second messenger, cyclic AMP, in a GTP-dependent manner in taste tissue homogenates. The cyclic AMP thus produced apparently stimulates a protein kinase A which may phosphorylate ion channels, leading ultimately to depolarization, an increase in intracellular calcium ion activity, and release of neurotransmitter. Recent studies show that some sweeteners also induce the production of inositol 1,4,5-trisphosphate (IP 3). Other sweet transduction processes not associated with second messenger production may exist. For example, evidence for a stimulus-gated type ion channel for sweet taste can be inferred from ion transport studies on lingual epithelia and from psychophysics. A recent study demonstrated that certain amphiphilic sweeteners are capable of directly stimulating purified G proteins in an in vitro assay. Perhaps these and other amphiphilic intense sweeteners cross the plasma membrane and directly stimulate the G protein, inducing production of second messenger and bypassing the receptor. A number of sweeteners are capable of forming ion channels or of simply perturbing the membrane, actions which could operate during stimulation of the sweet receptor cell. This type of action could explain the relatively longer response times and lingering taste intensity associated with many amphiphilic sweeteners.