title: Mechanisms of psoralen photosensitization reactions.
abstract: The basic aspects of cutaneous photosensitization reactions and the mode of therapeutic effectiveness of psoralens used extensively in the photochemotherapy of psoriasis and vitiligo have been reviewed. Psoralen-induced skin photosensitization and the therapeutic action of psoralens involve two distinct types of reactions that occur independently of each other and concurrently when the psoralen-treated skin (oral or topical) is exposed to 320-400 nm UV radiation. The first, type I, occurs in the absence of oxygen (anoxic) reaction and primarily involves photoreactions with DNA; the second, type II, is a sensitized reaction dependent on oxygen and involves the formation of singlet oxygen (1O2), superoxide anion (O-.2), and hydroxy radicals. The photoreactive form of psoralen is its triplet state, and the sites of reaction are 1) the cell membrane of the epidermal, dermal, and endothelial cells; 2) the cytoplasmic constituents, such as enzymes, RNA, lysosomes, etc.; 3) the cell nuclei (DNA and chromatin); and 4) psoralen-sensitized production of 1O2, O-.2, and hydroxy radicals, which we believe are responsible for cell membrane damage and vasodilation. The major damage would be initiated by a type I reaction and would be seen in the form of nuclear damage to DNA resulting from the interaction of psoralen with DNA and to a lesser extent with RNA. The skin photosensitization response (erythema, edema, membrane damage, etc.) would result from a type II reaction involving the generation of 1O2. In vitro and in vivo metabolism of 4,5',8-trimethylpsoralen and 8-methoxypsoralen is briefly outlined.
mesh_terms: D004247:DNA / Q000378:metabolism; D011564:Furocoumarins / Q000633:toxicity*; D006801:Humans; D008730:Methoxsalen / Q000378:metabolism; D010100:Oxygen; D010777:Photochemistry; D010787:Photosensitivity Disorders / Q000139:chemically induced*; D026082:Singlet Oxygen; D012867:Skin / Q000378:metabolism; D014307:Trioxsalen / Q000378:metabolism
chemical_list: D011564:Furocoumarins; D026082:Singlet Oxygen; D004247:DNA; D010100:Oxygen; D008730:Methoxsalen; D014307:Trioxsalen