The antioxidant role of astaxanthin and its biosynthesis in the yeast Phaffia rhodozyma

Phaffia rhodozyma is a heterobasidiomyceteous yeast that was originally isolated from the slime fluxes of birch tree wounds in mountainous regions of northern Japan and southern Alaska. Phaffia produces astaxanthin as its principal carotenoid pigment. Isolation of stable astaxanthin hyperproducing strains, for commercial purposes, will be facilitated by the design of positive genetic selections for enhanced astaxanthin biosynthesis.; The goal of this research was to investigate the functions of astaxanthin in Phaffia and to develop selections for astaxanthin production. The highly specific habitat of Phaffia, slime fluxes of wounded Betula (birch) trees, contained a compound(s) that selected for astaxanthin synthesis in the yeast. These compounds, extracted with ethyl acetate, produce singlet oxygen ({dollar}sp1{dollar}O{dollar}sb2{dollar}) when excited by ultraviolet light ({dollar}lambda{dollar} = 366 nm). Astaxanthin, a catalytic detoxifier of {dollar}sp1{dollar}O{dollar}sb2{dollar}, protected Phaffia from photoactivated birch extracts. Photogenerators of {dollar}sp1{dollar}O{dollar}sb2{dollar} such as rose bengal and {dollar}alpha{dollar}-terthienyl were also effective selective agents. Dark chemical formation of {dollar}sp1{dollar}O{dollar}sb2{dollar} either by the reaction of hydrogen peroxide (H{dollar}sb2{dollar}O{dollar}sb2{dollar}) with hypochlorous acid (HOCl) or by ozone decomposition also selected for astaxanthin hyperproduction.; Exposure to {dollar}sp1{dollar}O{dollar}sb2{dollar} increased carotenoid levels up to 40%. This induction only slightly changed the carotenoid composition. Exposure to a superoxide radical (O{dollar}sb2{dollar}-) generator increased total carotenoid up to 80% and increased the relative proportion of astaxanthin and other xanthophylls. Peroxyl radicals were found to degrade cellular carotenoids. Carotenoid degradation relieved feedback inhibition of carotenogenesis by astaxanthin.; The activities of the antioxidant enzymes superoxide dismutase (SOD) and catalase were investigated. Phaffia contained only the Mn-SOD form of the three known SOD isozymes (Fe, Mn, Cu/Zn) and contained low levels of catalase. This suggested that carotenoids may play a principal role in protecting against high levels of O{dollar}sb2{dollar}- and H{dollar}sb2{dollar}O{dollar}sb2{dollar} in the cell. Respiration became almost totally CN{dollar}sp{lcub}-{rcub}{dollar}resistant as Phaffia entered stationary phase, the growth stage when astaxanthin synthesis is maximum. CN{dollar}sp{lcub}-{rcub}{dollar}resistant respiration has been shown in other organisms to correlate with increased production of O{dollar}sb2{dollar}-.; Carotenoids provide Phaffia rhodozyma with antioxidant protection against toxic oxygen species produced during cellular metabolism or present in the environment.