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硫代葡萄糖苷(glucosinolates,GS),简称硫苷,又称芥子油苷,是植物中一类富含氮硫的阴离子次生代谢物质,主要存在于十字花科Cruciferae,尤其是芸薹属Brassica植物中,如白菜Brassica rapa ssp. pekinensis,甘蓝Brassica oleracea,油菜Brassica napus,芥菜Brassica juncea,芜菁Brassica rapa,拟南芥Arabidopsis thaliana等[1]。自从BUSSY[2]于1839年从芥菜子中首次发现硫苷后,硫苷的种类以及降解产物逐渐被人所认识。目前,鉴定出结构的硫苷已经超过132种[3]。所有硫苷都有一个共同的化学结构:一般由β-D-硫葡萄糖基、硫化肟基团以及来源于氨基酸的侧链R基团组成。根据氨基酸侧链R基团的不同,可将硫苷分为3类:脂肪族硫苷(侧链主要来源于甲硫氨酸、丙氨酸、缬氨酸、亮氨酸或异亮氨酸),吲哚族硫苷(侧链主要来源于色氨酸)和芳香族硫苷(侧链主要来源于苯丙氨酸或酪氨酸)[1, 4]。硫苷本身性质比较稳定,并不具备生物活性,主要存在于植物细胞的液泡中,而硫代葡萄糖苷酶(又称黑芥子酶)则位于特定的蛋白体中,只有当植物组织破碎时(如病虫害侵袭或机械损伤),两者得以接触,硫苷在黑芥子酶的作用下水解产生异硫氰酸盐、硫氰酸脂、腈类等生物活性物质[4]。这些水解产物具有重要的生物学功能,不仅是十字花科蔬菜独特风味物质的主要来源,而且在抵御昆虫取食[5-7]、病原菌侵染[8]以及各种非生物胁迫[9](如水分、温度、光照、盐胁迫)等植物防卫反应中也发挥了重要作用,更重要的是它对人体而言具有预防结肠癌、乳腺癌、肺癌等癌症发生的作用[10-11]。经过数十年的研究,硫苷的生物合成途径及其调节基因在模式植物拟南芥中已经基本阐明[12-14]。硫苷的生物合成过程主要包括以下3个阶段:氨基酸侧链的延长,核心结构的形成和侧链的次级修饰[12]。在硫苷核心结构形成过程中,硝基化合物或氧化腈在谷胱甘肽硫转移酶(gultathione-S-transferase,GST)的作用下与硫供体(半胱氨酸或谷胱甘肽)结合,形成S-烷基硫代氧肟;以及脱硫硫苷在磺基转移酶(sulfotransferase,SOT)的催化下,与高能硫供体3′磷酸腺苷5′磷酰硫酸(3′-phospho-adenosine-5′-phosphosullfate,PAPS)结合,在N末端生成一个SO42-,从而形成基本的硫苷结构。这2步反应都需要硫供体,也使得最终的硫苷中含有大量的硫元素且被运送到种子中储存起来,用于应对缺硫胁迫,保证植物体内的硫平衡[15]。笔者总结了近年来硫苷生物合成过程中硫来源的研究进展,并在此基础上分析了初生硫代谢与硫苷合成的关系,希望进一步完善硫苷的代谢网络,为日后研究硫的初生与次生代谢途径间的相互作用提供理论指导。
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