Participation of the photosensitizer alpha-terthienyl in the peroxidase-catalyzed oxidation of indole-3-acetic acid

The plant photosensitizer alpha-terthienyl (alpha T) is toxic toward a variety of organisms, and normally requires exposure to ultraviolet-A radiation for activation and singlet molecular oxygen formation. However, some toxicity has also been reported to occur in the dark. One hypothesis that has been proposed to account for this light-independent toxicity is that the sensitizer becomes activated by energy transfer from the excited-state products of enzymatic reactions. We have investigated this hypothesis using the horseradish peroxidase (HRP)-catalyzed oxidation of indole-3-acetic acid (IAA), which generates indole-3-aldehyde in an excited triplet state. Light is emitted during the IAA/HRP reaction at acidic pH, is increased by inclusion of alpha T and is not observed with heat-denatured HRP. The rates of both the oxidation of IAA and the subsidence of light emission are more rapid in the IAA/alpha T/HRP system than with IAA and HRP alone, indicating that the presence of alpha T accelerates the reaction. Bleaching occurs at the wavelength of maximal alpha T absorbance and is promoted by the inclusion of IAA. Readdition of both IAA and alpha T to a spent reaction mixture is required to restore light emission after it has subsided, further suggesting that both are consumed in the reaction. We were unable to detect measurable quantities of singlet molecular oxygen formation in this system. These results do not support the energy transfer hypothesis, but instead are more compatible with a model proposed by Krylov and Chebotareva [Krylov, S. N. and A. B. Chebotareva (1993) FEBS Lett. 324, 6-8] for the co-oxidation of IAA and xanthene dyes.