Stereospecificity of the sensory irritation receptor for nonreactive chemicals illustrated by pinene enantiomers

Abstract

To clarify the existence of a receptor protein for sensory irritants in trigeminal nerve endings, d- [i.e. (+)] and l- [i.e. (−)] enantiomers of α- and β-pinene as models of nonreactive chemicals were evaluated for their potency in outbred OF1 and NIH/S mice using ASTM E981-84 bioassay. All pinenes possess sensory irritation properties and also induced sedation and signs of anaesthesia but had no pulmonary irritation effects. According to the ratio of RD₅₀ (i.e. concentration which causes a 50% decrease in respiratory rate, f ) and vapour pressure (P^o), all pinenes are nonreactive chemicals. For nonreactive chemicals, P° and olive oil-gas partition (L_Oil) can be used to estimate their potency as sensory irritant. Thus, for enantiomers with identical physicochemical properties, the estimated RD₅₀ values are the same. In addition, although α- and β-pinene do not have identical P^o and L_Oil values, their estimated potencies are quite close. However, the experimental results showed that d-enantiomers of pinenes were the most potent as sensory irritants and a difference in potency also exists between α- and β-pinene. RD₅₀ for d-enantiomers of α- and β-pinene were almost equal, 1053 ppm and 1279 ppm in OF1 strain and 1107 ppm and 1419 ppm in NIH/S strain, respectively. Values differed by a factor of ∼4 to 5 from l-β-pinene for which the RD₅₀ was 4663 ppm in OF1 and 5811 ppm in NIH/S mice. RD₅₀ could not be determined for l-α-pinene; this pinene was almost inactive. d-α-pinene seems to best fit the receptor because its experimental RD₅₀ was one-half of the estimated value while for d-β-pinene those values were equal. On the contrary, l-β-pinene was about 3 to 4␣times less potent than estimated. l-α-pinene was only slightly active although it was estimated to be as potent as d-α-pinene. The remarkable difference in potency between l-enantiometers is most likely due to a structural difference between α- and β-pinene: the more flexible β-pinene can bend to fit into the receptor better than the rigid α-pinene. The results showed that the commonly used physicochemical descriptors cannot fully explain the potency of these chemicals; their three-dimensional structure should also be considered. Because of the stereospecificity of pinenes, a target site for nonreactive sensory irritants is most likely a receptor protein containing a chiral lipophilic pocket.