Microalgal AstaP is a natural solubilizer of astaxanthin with broad binding ability for carotenoids and related compounds

Abstract

Astaxanthin-binding water-soluble proteins, named AstaPs, were identified as photooxidative-stress inducible aqueous carotenoprotein found in microalgae of the family Scenedesmaceae. Although AstaPs bind astaxanthin, the properties of selective binding of astaxanthin remain unclear. In this study, we prepared carotenoid-free apo-AstaPs and investigated the ability to bind carotenoids. Apo-AstaP-orange1 selectively bound carotenoids, especially astaxanthin, from the pigments extracted from stressed cells containing secondary carotenoids, chlorophylls, and β-carotene. Reconstitution assays revealed that apo-AstaP-orange1 showed higher affinity for binding astaxanthin than the other tested carotenoids. Other AstaP family proteins, such as AstaP-orange2 and AstaP-pink1 from Scenedesmus obtusus Oki-4N, and recombinant AstaP-pink1 expressed in E. coli, also displayed a preference for binding astaxanthin. Apo-AstaP-orange1 bound astaxanthin and astaxanthin-monoesters but not astaxanthin-diesters, indicating that the protein requires at least one non-esterified β-end group for binding of the carotenoids. On mixing in vitro, apo-AstaP-orange1 also had the ability to bind lutein, adonixanthin, canthaxanthin, and other tested xanthophylls and carotenoid derivatives such as retinoids. Our results indicate that AstaPs are selective solubilizers of astaxanthin in vivo and potential natural solubilizers of carotenoids and carotenoid derivatives in vitro.