Mooser G
Proc Natl Acad Sci U S A. 1980 Mar;77(3):1686-90. doi: 10.1073/pnas.77.3.1686.
Stimulation of taste receptors with sodium chloride, sodium acetate, sodium propionate, and the respective potassium salts gave concentration-response profiles, measured electrophysiologically, which are remarkably consistent with a two-state allosteric mechanism. The allosteric constant or equilibrium constant for the transition between the active and inactive receptor states is low, resulting in a condition in which small differences in ion affinities for the two states are sufficient to significantly alter the equilibrium. Receptor activators, such as sodium ion, displaced the equilibrium toward the active receptor state by virtue of a higher affinity for that state, whereas receptor inhibitors, such as acetate and propionate ions, displaced the equilibrium in the opposite direction as a result of a higher affinity for the inactive state. The low allosteric constant increased about 10-fold after treatment with the protein modification reagent dimethyl(2-hydroxy-5-nitrobenzyl)sulfonium bromide, resulting in a marked reduction in the response to sodium chloride and sodium propionate without a significant change in ion affinities. In order to fully resolve the potassium response characteristics, it was necessary to consider both a potassium activation site and a potassium inhibition site. Analysis of the response from sodium chloride/potassium chloride mixtures showed that sodium ion is competitive with potassium binding at the activation site but not the inhibition site. With potassium propionate as the stimulus, the effect of both a receptor activator and a receptor inhibitor was quantitatively consistent with depression of the response below a water baseline level at low stimulus concentrations. Estimation of active and inactive state dissociation constants for each anion and cation permitted accurate prediction of the response magnitude for a range of cation ratios in sodium chloride/potassium chloride mixtures and anion ratios in sodium chloride/sodium propionate mixtures. The association of salty taste with receptor activators and bitter taste with receptor inhibitors may be relevant to the generation of these taste qualities.
用氯化钠、醋酸钠、丙酸钠以及各自对应的钾盐刺激味觉感受器，通过电生理测量得到的浓度-反应曲线，与双态变构机制显著一致。活性受体状态和非活性受体状态之间转变的变构常数或平衡常数很低，这导致了一种情况，即两种状态下离子亲和力的微小差异就足以显著改变平衡。受体激活剂，如钠离子，由于对活性受体状态具有更高的亲和力，使平衡向活性受体状态移动，而受体抑制剂，如醋酸根离子和丙酸根离子，由于对非活性状态具有更高的亲和力，使平衡向相反方向移动。用蛋白质修饰试剂二甲基(2-羟基-5-硝基苄基)溴化锍处理后，低变构常数增加了约10倍，导致对氯化钠和丙酸钠的反应显著降低，而离子亲和力没有明显变化。为了完全解析钾的反应特性，有必要同时考虑钾激活位点和钾抑制位点。对氯化钠/氯化钾混合物反应的分析表明，钠离子在激活位点与钾离子结合存在竞争，但在抑制位点不存在竞争。以丙酸钾作为刺激物，受体激活剂和受体抑制剂的作用在低刺激浓度下，都与反应低于水基线水平的抑制作用在数量上是一致的。对每种阴离子和阳离子的活性和非活性状态解离常数的估计，使得能够准确预测氯化钠/氯化钾混合物中一系列阳离子比例以及氯化钠/丙酸钠混合物中阴离子比例下的反应幅度。咸味与受体激活剂的关联以及苦味与受体抑制剂的关联，可能与这些味觉品质的产生有关。
