A LARGE variety of biological systems are susceptible to damage by light and oxygen in the presence of a suitable sensitizer. Such photosensitized oxidations in biological systems are termed photodynamic action (for review, see [l]). Largely as the result of the work of Foote and coworkers[2], it is now believed that many photosensitized oxidations proceed via a mechanism suggested by Kautsky [3,4], involving excited singlet molecular oxygen. These latter results have led to the additional suggestion that singlet oxygen may often be involved in photodynamic action [5,61. We have shown earlier[7-10] that a number of exothermic reactions of ozone with organic compounds are sources of singlet oxygen. Because many of these reactions occur under very mild conditions and offer a very efficient source of singlet oxygen, we are attempting to use them to examine the possible role of singlet oxygen in photodynamic action. Since the singlet oxygen so produced does not require application of radiation, we hope to be able to isolate products which might be further transformed under photosensitizing conditions.In one such attempt, we have shown that triphenyl phosphite ozonide, an efficient source of singlet oxygen, smoothly oxidizes simple dialkyl disulfides to the corresponding thiolsulfinates [ 1 11. To examine further the mechanism of this reaction, we have also subjected the same disulfides to photosensitized oxidation with methylene blue [ 121. These reactions also smoothly convert the disulfides to the corresponding thiolsulfinates.The disulfide bond plays an enormously important role in biological systems (for an excellent review, see [ 13]), particularly in maintaining specific conformations required for the activity of many proteins, enzymes, and hormones. Our results suggested that photodynamic action on biological substrates containing the cystine residue, for example, could oxidize the disulfide bond, with biological function being impaired or destroyed.Weil, Gordon, and Buchert had originally reported[l4] that cystine is photooxidized in aqueous medium in the presence of methylene blue. More recently, however, Weil has reported that cystine reacts only sluggishly in photosensitized oxidation [ 151. Likewise it has been reported that the photosensitized oxidation of bovine insulin at 10" is confined to the histidine residues [ 161. At higher temperatures, tyrosine residues are attacked, but no evidence is found for oxidation of cystine residues.Caution must be exercised, however, in extrapolating from the results with cystine 147
