Abstract

For some years the mechanism of the photosensitizing effects displayed by some furocoumarins on various biological substrates (human and guinea-pig skin, bacteria cultures, mammalian cells adapted to in vitro growth, viruses) have been studied. Recently it has been pointed out that a photoreaction occurs between the photosensitizing furocoumarins and DNA after irradiation at 3655 A. By use of a labeled furocoumarin, i.e.-O"CH, bergapten or 5-methoxy-psoralen, this has been confirmed and more extensively studied. During the irradiation a stable combination of the furocoumarin with native DNA takes place with a quantum yield of 5.2 x lo-*. It is probable that the reactive sites of DNA are the pyrimidine bases. Yeast-RNA and the same DNA after heat-denaturation or in the presence of high NaCl concentration photoreact at a much reduced rate. This photoreaction may explain some various biological photosensitizing effects produced by furocoumarins. I N T R O D U C T I O NTHE FUROCOUMARINS, or psoralens, have been extensively studied in this and by American since some of these substances (mainly psoralen and various methyl-derivatives, xanthotoxin or 8-methoxy-psoralen, bergapten or 5-methoxy-psoralen, among others) have a photosensitizing action on the human or guinea-pig skin, i.e. they induce erythema followed by pigmentation. They act also, upon irradiation at 3655 A, on cultures of microorganism, with a letha1(7.8) and mutagenic on the mammalian cells adapted to in vitro growth with the formation of giant cells,(lO) and they inactivate DNA-containing viruses. (11)In continuing our investigation of the mechanism of action of the skin-photosensitizing furocoumarins, we have recently reported(llJ2) studies on the interaction between these substances and deoxyribosenucleic acid (DNA).We have pointed out that all furocoumarins, both skin-photosensitizing and inactive, have the capacity of binding in darkness to DNA and, to a much less extent, to RNA.('')After irradiation of an aqueous solution of DNA and a skin-active furocoumarin at 3655 A, we have observed strong modifications in the fluorescence spectra of the solutions, whereas in the presence of skin-inactive substances the spectra remained unchanged. (12)

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