THE UV RADIATION-DRIVEN REDUCTION OF PTERINS IN AQUEOUS-SOLUTION

This paper demonstrates the capability of pterins, a strongly suspecte d photoreceptor in W-governed physiological and developmental processe s, to undergo redox transitions when excited by UV radiation. Aqueous solutions of 2-amino-4-hydroxy-6,7-dimethyl-pteridine (DMP) and 4-hydr oxy-6-tetrahydroxy-butyl-(D-arabo)-pteridine (TOP) were anaerobically irradiated with the total spectrum from a high pressure Hg lamp or in the range 290-390 nm cut off by absorption filters. Changes in the ele ctronic spectra of the pterins were indicative of their reduction afte r irradiation in the presence of Na-2-EDTA. Kinetic analysis, together with a direct demonstration of the photoreduction of DMPH2, revealed a stepwise transformation of DMP to the final product DMP -->(k1) DMPH 2 -->(k2) DMPH4 The rate of DMP conversion into DMPH2 was the same for both neutral and anionic molecules, whereas for the conversion from D MPH2 to DMPH4, anionic molecules were four times more active than neut ral molecules. The structure of the residue at position 6 of the pteri dine heterocycle influenced the photoreaction: the rate constant for T OP reduction exceeded by 2.5 times the k(1) value for DMP. Since the c oncentration of KI required to quench the fluorescence from DMP and TO P excited singlet states was found to be at least one order of magnitu de higher than that required to inhibit their photoreduction, the invo lvement in the reaction of the triplet state of the pterins rather tha n the excited singlet state was presumed. (C) 1997 Elsevier Science S. A.