Chemostimulatory protein: a new type of taste stimulus.
Three taste-active proteins have recently been discovered. It is proposed that two of these (monellin and thaumatin) should be classified as chemostimulatory proteins because of their sensory effect; these two proteins taste intensely sweet. The third protein (miraculin), a taste-modifier protein, changes the normal sour taste of acids to sweet. The taste-modifier protein, miraculin, occurs in the fruit of the tropical plant Synsepalum dulcificum. Though itself not sweet, it is able to change the taste of acids from sour to sweet after the tongue has been treated with the protein. Miraculin is a basic glycoprotein with a molecular weight of 44,000. Monellin, a chemostimulatory protein, is found in the fruit of a different tropical plant, Dioscoreophyllum cumminsii. It has been characterized as a basic protein with a molecular weight of 10,700 that contains no carbohydrate. Thaumatin, another chemostimulatory protein, occurs in the fruit of a third tropical plant, Thaumatococcus daniellii. Like monellin, it is a basic protein that contains no carbohydrate. Its molecular weight is around 21,000. Certain gross similarities among the three proteins have been noted. Their basic ionic character and some features of the amino acid compositions are similar. Little is known of the structural features of the chemostimulatory proteins that are required for eliciting their intense sweetness; they are of the order of 10(5) times more effective than sucrose. The precise role of the tertiary structure in their biological activity is not known but appears to be an important area for further study. The relatively large size (11,000 to 21,000 molecular weight) of the chemostimulatory proteins provides indirect evidence that the initial interaction of these stimuli with taste receptor cells occurs at the plasma membrane.