微观层面中的香气协同作用现象

Buck和Axel的研究表明，嗅觉系统能识别和分辨大量不同的气味分子，起始于嗅神经元纤毛上的嗅觉受体与气味分子的相互作用。香气的识别可以是一个或者多个嗅觉受体作用的结果。嗅觉受体具有一定数量的活性位点。因此，香气物质须采用竞争或者非竞争方式占据作用位点。而不同香气成分与受体活性位点的结合能力存在差异。当不同香气成分之间存在竞争关系时，往往表现为掩盖作用；不同香气成分之间存在相互协作或者非竞争关系时，往往表现为协同作用。直接研究香气物质与活性位点的作用比较困难，但是可以研究嗅神经元对香气物质刺激的响应特性，监测细胞电生理变化等手段，可间接研究香气成分之间的相互作用。

香气成分之间的相互作用模式与成分浓度有关。在低浓度或者中等浓度时，香气成分之间更容易表现为协同或者加成作用；高浓度时，香气成分之间往往具有掩盖、抑制作用。Fouzia EI Mountassir等在采用神经元钙离子成像技术，研究不同浓度单一和混合香气成分（辛醛、3-甲硫基丙醛、香茅醛）对4种嗅觉受体蛋白（OR1G1、OR52D1、OR1A1、OR2W1）的激活效果时，监测了钙离子的浓度变化，计算刺激条件下细胞的响应个数，并以σ-τ图形式判定香气成分之间相互作用。结果表明，含有4种受体蛋白的细胞对辛醛与高浓度香茅醛混合的刺激，钙离子浓度发生改变，且细胞响应个数低于单个成分刺激反应的总和，即发生了掩盖或加成作用。M.A.Chaput等研究红酒中乙酸异戊酯和威士忌内酯与OR1G1受体的相互作用。相比乙酸异戊酯的单一成分，加入高浓度的威士忌内酯时，OR1G1神经元活性明显减弱，即乙酸异戊酯和威士忌内酯发生了掩盖作用。而低浓度时，乙酸异戊酯和威士忌内酯发生了协同作用。Patricia Duchamp-Viret等以小白鼠为研究对象，考察了甲位突厥酮、铃兰醛、新铃兰醛等香气成分给予嗅觉刺激时，神经元的电生理反应，并绘制浓度与神经元响应曲线。结果表明，甲位突厥酮和铃兰醛的混合体系在阈值浓度和中等浓度条件下，响应曲线的斜率大于甲位突厥酮、铃兰醛单一成分斜率，故呈现为协同作用。

从上面的研究来看，当不同香气成分相互协作或者互不干扰占据位点时，更容易发生协同作用；不同香气成分相互竞争活性位点时，更容易发生掩盖作用。而当神经元去极化作用达到最大化、受体蛋白饱和时，香气成分之间协同作用向加成或者掩盖作用转化。这就解释了香气成分在低浓度下为协同作用，高浓度下为加成或掩盖作用的机制了。

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