20 V. G. DETHIER 



of salt above which no increase of sugar will effect a balance. This "saturation 

 level" is different for different compounds. 



On the basis of some of the facts given in the preceding paragraphs and by 

 making certain basic assumptions Beidler (1954) has proposed a theory of taste 

 applicable to salt receptors. He has assumed that the stimulus reacts with some 

 receptor substance and that the reaction obeys the mass action law. His second 

 assumption is that the magnitude of response is directly related to the number 

 of ions or molecules that have reacted with the receptors. An equation can be 

 derived which relates magnitude of response to the concentration of the applied 

 chemical stimulus. This is c/ A = (c/R m ) + (l/KR m ) where c = concentration, 

 R = magnitude of response, R m — magnitude of maximum response, and A' = 

 the equilibrium constant. The validity of the equation can be tested by meas- 

 uring c and R. Beidler found the agreement to be excellent. Now, knowing the 

 equilibrium constant, he calculated the free energies of reaction from the ex- 

 pression AF = RT In A' where AF = change of free energy, R = the gas con- 

 stant, and T the absolute temperature. The low value which he found for AF 

 (from —1.22 to —1.37 Cals./mole) for a series of sodium salts is taken to indi- 

 cate that physical rather than chemical forces are involved in the interaction 

 between the chemical stimulus and the receptors. This conclusion raises the 

 question of what the receptor substance may be. 



Beidler found that the magnitude of response in receptors of the rat varied 

 little with change of temperature. Earlier studies of the effects of temperature 

 on human taste thresholds had shown little consistency from one worker to the 

 next or for any two substances. In this connection Pfaffmann (1951) had con- 

 cluded that the temperature effects were complex and not indicative of a simple 

 chemical reaction. The findings with respect to insects generally failed to show 

 any convincing temperature effect on threshold (Dethier and Chadwick, 1948a). 

 Beidler has argued that the small temperature dependence and the low values 

 for AF suggest a reaction similar to those which occur with ion binding by pro- 

 teins and natural polyelectrolytes. Some properties of the reacting groups of the 

 receptor substance can be determined from a study of the effect of changing pH. 

 Since no such change could be demonstrated over the range 3.0 to 11.0, one 

 might conclude that the molecules of the receptor substance are strong acidic 

 radicals. The relatively weak carboxyl radical of a protein, for example, cannot 

 be considered as the reacting group. The phosphate and sulfate radicals of such 

 natural polyelectrolytes as nucleic acids and certain polysaccharides are able 

 to bind cations in a manner consistent with the properties of the receptors de- 

 scribed (Beidler, 1954). According to Beidler the ability of taste receptors to 

 respond to a variety of cations would vary with the nature of the reacting radical 

 and with the proximity of neighboring reacting groups as well. Differences in 

 responses of taste substances as already demonstrated by Beidler, Fishman, 



