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于动物、植物和微生物中的天然色素。然而，各种类胡萝卜素种类的化学结构和体内保护活性之间的相关性很少被研究。本研究以紫色光合细菌Rubrivivax（Rvi.）gelatinosus：在构建突变株之前，Rvi.明胶，这一直是不清楚的，是确定的。由于八氢番茄红素去饱和酶CrtI的特性特异性，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。在氧气存在下，两种类型的酮类胡萝卜素也合成为最终产品的类胡萝卜素单加氧酶，CrtA。基于阐明的生物合成途径，突变株的Rvi。构建了积累特定类胡萝卜素种类的胶质菌，通过Rvi每个突变体的活力来评估每个类胡萝卜素种类对光氧化损伤和单线态氧的保护活性。在有氧高光条件下或在单线态氧存在下，在类胡萝卜素种类中，酮基类胡萝卜素、球胡萝卜素和二酮基胡萝卜素具有显著的清除单线态氧的活性。具有长共轭双键的螺菌黄质和番茄红素对单线态氧的保护作用低于酮类胡萝卜素，尽管它们对光氧化损伤表现出足够的保护作用。

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.