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1 January 2005 Enhancement of Riboflavin-mediated Photo-Oxidation of Glucose 6-phosphate Dehydrogenase by Urocanic Acid
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We have investigated the riboflavin (RF)-sensitized inactivation of glucose 6-phosphate dehydrogenase (G6PD) in the presence and absence of trans-urocanic acid (UCA). The inactivation of the enzyme results from its direct oxidation by the excited triplet RF in a Type-I–photosensitized reaction whose efficiency increases at low oxygen concentration. The addition of histidine to the system produced no change in the inactivation rate, discarding the participation of singlet oxygen in the reaction. On the other hand, the presence of UCA results in its bleaching, with a significant enhancement of RF-mediated inactivation of G6PD. Both the consumption of UCA and G6PD are faster at low oxygen concentrations. UCA also produced a decrease in the sensitizer photodecomposition yield. These results indicate that the enhancement of the RF-mediated G6PD inactivation observed in the presence of UCA is not a singlet oxygen–mediated process. It is proposed that UCA consumption and its effect on G6PD inactivation are due to a complex reaction sequence initiated by a direct oxidation of UCA by the excited sensitizer triplet. The oxidation of the semireduced flavin gives rise to reactive oxygen species (ROS) responsible for the increased rate of the process. This is supported by the protection afforded by several additives with ROS removal capacity: benzoate, superoxide dismutase and catalase.

Eduardo Silva, Leonardo Herrera, Ana María Edwards, Julio de la Fuente, and Eduardo Lissi "Enhancement of Riboflavin-mediated Photo-Oxidation of Glucose 6-phosphate Dehydrogenase by Urocanic Acid," Photochemistry and Photobiology 81(1), 206-211, (1 January 2005).
Received: 14 July 2004; Accepted: 1 October 2004; Published: 1 January 2005

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