Bombardier beetles are well-known for their remarkable defensive mechanism. Their defensive apparatus consists of two compartments known as the reservoir and the reaction chamber. When challenged, muscles surrounding the reservoir contract sending chemical precursors into the reaction chamber where they mix with enzymes resulting in an explosive discharge of a hot noxious chemical spray containing two major quinones: 1,4-benzoquinone and 2-methyl-1,4-benzoquinone (toluquinone). Previously, it has been speculated that the biosynthesis of all benzoquinones originates from one core precursor, 1,4-hydroquinone. Careful ligation of the base of the reservoir chamber enabled us to prevent the explosive reaction and sample untransformed reservoir fluid, which showed that it accumulates significant quantities of 1,4-hydroquinone and 2-methyl-1,4-hydroquinone. We investigated the biosynthetic mechanisms leading to quinone formation by injecting or feeding Brachinus elongatulus beetles with stable-isotope-labeled precursors. Chemical analysis of defensive secretion samples obtained from 1,4-hydroquinone-d₆-administered beetles demonstrated that it underwent conversion specifically to 1,4-benzoquinone. Analogously, results from m-cresol-d₈ injected or fed beetles confirmed that m-cresol is metabolized to 2-methyl-1,4-hydroquinone, which is then oxidized to 2-methyl-1,4-benzoquinone in the hot spray. Our results refute the previous claim that 1,4-hydroquinone is the precursor of all substituted benzoquinones in bombardier beetles and reveal that they are biosynthetic products of two independent pathways. Most likely, the aforementioned biosynthetic channel of hydroxylation of appropriate phenolic precursors and subsequent oxidation is not restricted to bombardier beetles; it could well be a general pathway that leads to the formation of all congeners of benzoquinones, one of the most widely distributed groups of defensive compounds in arthropods.

Graphical abstract

中文翻译：

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庞巴迪甲虫Brachinus elongatulus爆炸性排放中苯醌的生物合成来源。

庞巴迪甲虫以其卓越的防御机制而闻名。他们的防御装置包括两个被称为储藏室和反应室的隔室。当受到挑战时，储层周围的肌肉收缩，将化学前体送入反应室，在其中它们与酶混合，从而爆炸性排出含有两种主要醌：1、4-苯醌和2-甲基-1,4-的热毒化学喷雾。苯醌（甲苯醌）。以前，已经推测所有苯醌的生物合成都来自一种核心前体1,4-氢醌。小心地结扎储液室底部使我们能够防止爆炸反应和对未转化的储液进行采样，这表明它积聚了大量的1,4-对苯二酚和2-甲基-1，4-氢醌。我们研究了通过注射或进食导致醌形成的生物合成机制带有稳定同位素标记前体的长臂腕甲虫。从1,4-对苯二酚d ₆施用的甲虫获得的防御性分泌物样品的化学分析表明，它专门经历了向1,4-苯醌的转化。类似地，从结果米-cresol- d ₈注入或送入甲虫证实米-甲酚被代谢为2-甲基-1,4-氢醌，然后在热喷雾中被氧化为2-甲基-1,4-苯醌。我们的结果驳斥了先前的主张，即1,4-氢醌是庞巴迪甲虫中所有取代的苯醌的前体，并揭示了它们是两条独立途径的生物合成产物。最可能的是，上述适当的酚类前体的羟基化和随后的氧化的生物合成通道不限于庞巴迪甲虫；它很可能是导致所有同类物苯醌生成的一般途径，这是节肢动物中分布最广泛的防御性化合物之一。

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