A possible mechanism of psoralen phototoxicity not involving direct interaction with DNA.

Psoralens in combination with ultraviolet light (UVA; 320-400 nm) are used in the photochemical treatment of a variety of skin diseases including vitiligo, a skin depigmentational disorder, and psoriasis, a disease of accelerated epidermal cell proliferation. Although it is generally assumed that the major site of action of the psoralens is DNA, we have obtained evidence that another site may be the primary target for these compounds. We have identified specific, saturable, high-affinity binding sites for 8-methoxypsoralen on HeLa cells and have detected specific binding of 8-methoxypsoralen to four other human cell lines and five mouse cell lines. In HeLa cells, specific binding is reversible and independent of the ability of the compound to intercalate into DNA. In addition, binding sites become covalently modified by the psoralen after UVA exposure. Specific binding of 8-[methoxy-3H]methoxypsoralen constitutes 79% of the label bound to the cells. Scatchard analysis indicated two classes of psoralen binding sites: high-affinity sites with a Kd of 19 X 10(-9) M (1.8 X 10(5) sites per cell) and low-affinity sites with a Kd of 4 X 10(-6) M (7.1 X 10(6) sites per cell). Four structurally related psoralen analogs block 8-methoxypsoralen binding in a manner that parallels their biological activity. Based on these findings, we hypothesize that specific binding sites for psoralens on mammalian cells mediate, at least in part, psoralen-induced phototoxicity.