[Mechanisms of psoralen photoreaction].

The molecular aspects of the mode of action of psoralens is not clearly understood since two types of reactions may occur simultaneously and independently of each other, when the psoralen-treated skin is exposed to ultraviolet A radiation. Type 1 is an oxygen-independent reaction, leading directly to photoaddition of psoralen to pyrimidine bases of DNA, and type 2 is an indirect oxygen-dependent reaction involving the generation of reactive oxygen species. Thus, the psoralens can damage the cell membranes, the cytoplasmic constituents and the cell nuclei resulting in the observed phototoxic effects. The molecular mechanisms underlying the generation of free radicals and DNA damages are described, and are related to the two classes of psoralens.