FORMATION OF A HYDROXYL RADICAL FROM TAR DYE BY PHOTO-ILLUMINATION

When indigo carmine (B-2) was illuminated in the presence of phenylala nine in 0.1 M citrate buffer (pH 4.0), p-tyrosine, m-tyrosine and o-ty rosine were identified as hydroxylated products. However, ten other fo od colors did not form tyrosine isomers. The hydroxylation of B-2 was pH-dependent, and the maximum rate was found at around pH 4.0. Replace ment of air with nitrogen gas completely prevented the formation of ty rosine isomers and the decomposition of B-2. In contrast, oxygen gas a ccelerated both the hydroxylation and the decomposition.The addition o f superoxide dismutase or catalase to this system prevented hydroxylat ion. Chemical scavengers of the hydroxyl radical (HO .) prevented the hydroxylation. On the other hand, a singlet oxygen scavenger had no si gnificant effect. The above results suggest that the formation of HO . may occur in the photochemical reaction system in the presence of B-2 under aerobic conditions, and that a superoxide radical and hydrogen peroxide may be involved in the HO formation.