Abstract
One common feature of radioresistant bacteria is that they contain red carotenoid pigments. Their colorless mutants are more sensitive than wild types to ionizing radiation. Therefore, these red carotenoid pigments are thought to contribute to defense mechanisms of radioresistant bacteria against ionizing radiation. Carotenoid pigments are localized within the lipids of the cell surfaces in prokaryotes. From these facts, we consider that red carotenoid pigments in radioresistant bacteria must defense the lipids of the cell surfaces of these bacteria against ionizing radiation. In this study, the effects of beta-carotene and astaxanthin, which are typical carotenoids, on the degradation and peroxidation of a biological lipid (linolenic acid) induced by gamma irradiation were analyzed. Linolenic acid was dissolved in benzene at a final concentration of 0.5 M, and the carotenoids were added at final concentrations of 5.0 x 10-8 to 8.5 x 10-3 M. The prepared solutions were irradiated with 60Co gamma-rays at a dose of 30 kGy. The effects of the carotenoids on the degradation of linolenic acid induced by gamma irraidation were analyzed by TBA reaction and measurement of the absorbance at 532 nm of the reaction solutions. The effects of the carotenoids on the peroxidation of linolenic acid induced by gamma irraidation were analyzed by measurement of the absorbance at 230-236 nm of the n-hexane solutions of the irradiated samples. In these experimental conditions, the relatively high concentrations of beta-carotene and astaxanthin inhibited the degradation of linolenic acid induced by gamma irradiation, while the relatively low concentrations of the carotenoids promoted the degradation of linolenic acid. The carotenoids did not affect the peroxidation of linolenic acid induced by gamma irraidation. These results suggest that carotenoid pigments are active in the prevention of damage to biological molecules after the formation of lipid peroxide induced by ionizing irradiation in radioresistant bacteria and that concentrations of the carotenoid pigments in the cells are strictly regulated.
