Stereospecificity of the Receptor Site for Glycerol, a New Sweetener, in a Labellar Sugar Receptor Cell of Drosophila

Abstract

graph. The concentration for each compound was 1.0 M, which is close to the maximum response for most chemicals. Note that none are more stimulative than glycerol itself. The glycerol site was characterized by comparing the effectiveness of various derivatives of glycerol. Based on this structure–taste relationship of glycerol, a model is proposed for the glycerol site including three subsites and two steric barriers, which cannot accommodate carbon-ringcontaining sugars such as D-glucose. Specificity of the glycerol site Our model suggests rigid specificity for the glycerol site. This specificity can be confirmed by approaches other than structure-taste relationship: through inhibitors, for example. In the course of examining the effectiveness of glycerol derivatives, it was found that 2-amino1,3-propanediol and 3-amino-1,2-propanediol are nonstimulative, but have a clear inhibitory effect on the response to glycerol. They inhibited the response to glycerol, specifically and competitively as compared with the almost total lack of effects of the reagent on the responses to the four sugars sucrose, D-glucose, D-fructose and trehalose. The concentration-response curves for glycerol were found to be indistinguishable for I-type and L-type hairs. The curves of the responses to these four sugars, however, were significantly different. The magnitude of each response from an L-type hair is statistically larger than that from an I-type hair. This difference in the curves for glycerol and the four sugars is compatible with the presence of a specific receptor site for glycerol.