Evaluation of carotenoid function by making mutants of a purple photosynthetic bacterium that accumulate specific carotenoid species

类胡萝卜素是广泛存在的天然色素，存在于动物、植物和微生物中。然而，各种类胡萝卜素的化学结构与体内保护活性之间的相关性研究很少。本研究利用紫色光合细菌Rubrivivax (Rvi.) gelatinosus的突变体，比较研究了几种类胡萝卜素在体内的光保护活性。在构建突变菌株之前，研究了Rvi的类胡萝卜素生物合成途径。之前不清楚的明胶被确定了。由于植物烯去饱和酶CrtI的特异性，该细菌的类胡萝卜素生物合成途径被证明由两条途径组成。在氧气存在的情况下，类胡萝卜素单加氧酶CrtA也合成了两种类型的类酮胡萝卜素作为最终产物。基于已阐明的生物合成途径，Rvi突变株。聚积了一种特殊的类胡萝卜素的明胶菌。利用Rvi突变体的活力评价了各类胡萝卜素对光氧化损伤和单线态氧的保护活性。在有氧强光条件下或单线态氧存在下的明胶鱼。在类胡萝卜素种类中，类酮胡萝卜素、球甾酮和二酮螺旋黄素具有显著的清除单线态氧的活性。螺旋黄质和番茄红素具有较长的共轭双键，对单线态氧的保护作用不如类酮胡萝卜素，但对光氧化损伤具有足够的保护作用。

Carotenoids are widespread natural pigments, occurring in animals, plants and microorganisms. Nevertheless, correlation between the chemical structure of various carotenoid species and the protective activity in vivo has rarely been investigated. In this study, photoprotective activities of several carotenoid species in vivo were comparatively investigated by using mutants of a purple photosynthetic bacterium, Rubrivivax (Rvi.) gelatinosus:Before constructing mutant strains, carotenoid biosynthesis pathway of Rvi. gelatinosus, which had been unclear, was determined. Carotenoid biosynthesis pathways of this bacterium were shown to be composed of two pathways due to characteristic specificity of the phytoene desaturase, CrtI. In the presence of oxygen, two types of ketocarotenoids were also synthesized as final products by a carotenoid mono-oxygenase, CrtA. Based on the elucidated biosynthesis pathways, mutant strains of Rvi. gelatinosus accumulating a particular carotenoid species were constructed.The protective activity of each carotenoid species against photooxidative damage and singlet oxygen was evaluated by the viability of each mutant of Rvi. gelatinosus under aerobic high-light condition or in the presence of singlet oxygen. Among the carotenoid species, ketocarotenoids, spheroidenone and diketospirilloxanthin, were shown to have remarkable activity of scavenging singlet oxygen. Spirilloxanthin and lycopene that have long conjugated double bonds showed less protectivity against singlet oxygen than ketocarotenoids, although they showed enough protectivity against photooxidative damage.