DIFFERENTIAL REACTIVITY OF BETA-CAROTENE ISOMERS FROM DUNALIELLA-BARDAWIL TOWARD OXYGEN RADICALS

Dunaliella bardawil accumulates massive amounts of beta-carotene in two isoforms, a 9-cis and an all-trans stereoisomer, when grown under high irradiance, as a means to protect the cells against photoinhibition (A. Ben-Amotz, A. Shaish, M. Avron [1989] Plant Physiol 91: 1040-1043). The purpose of this work has been to find out if the mechanism of protection involves scavenging of reactive oxygen species. For this purpose high- and low-beta-carotene-containing cells were compared with respect to their sensitivity to several external oxidants [H2O2, methyl viologen, rose bengal, and 2,2'-azobis(2-amidinopropane)HCl]. All oxidants induce a light-stimulated degradation of beta-carotene and of chlorophyll. The degradation of beta-carotene precedes that of chlorophyll, indicating that it is more reactive toward oxidants. The 9-cis beta-carotene is degraded faster than the all-trans stereoisomer when exposed to oxidants, both in intact cells and in isolated beta-carotene globules, indicating that it is a more effective scavenger of reactive oxygen species. Comparison of the sensitivity to different oxidants, between high- and low-beta-carotene-containing cells, reveals similar rates of chlorophyll and beta-carotene degradation in the two populations. Survival tests toward H2O2 and rose bengal show that high-beta-carotene cells have a similar sensitivity toward H2O2 but are more resistant toward rose bengal, a photoactivated generator of singlet oxygen, possibly due to masking of the latter by beta-carotene. These results suggest that the protection mechanism of massively accumulated beta-carotene in Dunaliella against photoinhibition is not due to scavenging of reactive oxygen species.