Bacillithiol is a major low-molecular-weight thiol in gram-positive firmicutes including the human pathogen Staphylococcus aureus. Bacillithiol is regarded as an important defence mechanism against oxidants produced by the immune system, especially myeloperoxidase-derived hypochlorous acid (HOCl). However, it is unknown how fast BSH reacts with HOCl and what products are formed in the reaction. In the present study, we used sensitive MRM-based LC-MS methods to characterize the reaction of BSH with HOCl in cell-free solutions and in S. aureus. In the cell-free system, BSH formed predominantly the disulfide dimer (BSSB) at low mole ratios of HOCl and the sulfinic and sulfonic acids at higher oxidant concentrations. HOCl also promoted the formation of bacillithiol sulfonamide. In S. aureus, the oxidation pattern was similar except that a small proportion of BSH also formed mixed disulfides with protein thiols.

Using competition with methionine, we determined the second-order rate constant for the reaction of HOCl with BSH to be 6 × 10⁷ M^-1s^-1, which indicated a fast, near diffusion-controlled reaction. Other reactive halogen species, including hypothiocyanous acid (HOSCN), also produced bacillithiol sulfonamide, albeit to a smaller extent than HOCl. The sulfonamide was not produced by hydrogen peroxide, which instead formed BSSB.

This study helps our understanding of BSH redox biology and provides tools for gauging the exposure of BSH-producing bacteria to oxidative stress.

芽孢杆菌硫醇是革兰氏阳性菌中主要的低分子量硫醇，包括人类病原体金黄色葡萄球菌。bacillithiol被认为是抵抗免疫系统产生的氧化剂的重要防御机制，尤其是源自髓过氧化物酶的次氯酸（HOCl）。但是，尚不清楚BSH与HOCl反应的速度如何以及在反应中形成了什么产物。在本研究中，我们使用了基于MRM的灵敏的LC-MS方法来表征无细胞溶液和金黄色葡萄球菌中BSH与HOCl的反应。在无细胞系统中，BSH主要以低摩尔比的HOCl和较高氧化剂浓度的亚磺酸和磺酸形成二硫键二聚体（BSSB）。HOCl也促进了杆菌硫醇磺酰胺的形成。在金黄色葡萄球菌的氧化模式相似，除了一小部分的BSH还会与蛋白质硫醇形成混合的二硫化物。

利用与蛋氨酸的竞争，我们确定HOCl与BSH反应的二级速率常数为6×10 ⁷  M ^-1 s ^-1，这表明反应是快速的，接近扩散控制的反应。其他反应性卤素物质，包括次硫氰酸（HOSCN），也产生了巴硫醇磺酰胺，尽管程度比HOCl小。磺酰胺不是由过氧化氢产生的，而是形成了BSSB。

这项研究有助于我们了解BSH氧化还原生物学，并提供了一些工具，可用来衡量产生BSH的细菌受到氧化应激的程度。