An electron microscopic study of the photochemical cross-linking of DNA in guinea pig epidermis by psoralen derivatives.
Albino guinea pigs were treated with psoralen derivatives plus 320--400 nm ultraviolet radiation, and DNA was extracted from their epidermis. The DNA was assayed for the presence of interstrand cross-links by standard denaturation-renaturation assays and by a new technique, electron microscopy of the DNA under totally denaturing conditions. The latter method allows individual cross-links to be directly observed and counted. When either 4,5',8-trimethylpsoralen or 8-methoxypsoralen was applied topically to the skin (8--20 microgram/cm2) or administered orally (10--12 mg/kg body weight), followed by exposure to 320--400 nm ultraviolet radiation, most of the epidermal DNA was found to contain a high frequency of cross-links. For example, oral or topical trimethylpsoralen treatment gave an average of one cross-link per 250 nucleotide pairs or about 3 . 10(5) cross-links per guinea pig chromosome. When the dose of either drug was decreased 20-fold to the level used in the clinical treatment of psoriasis, however, no cross-links coulld be detected in the epidermal DNA. The electron microscopic assay is sensitive enough that we can put an upper limit of 1 cross-link per 10(6) nucleotide pairs (80 cross-links per chromosome) for the low dose studies. The significance of these findings to the understanding of the effectiveness of psoralens in psoriasis therapy is discussed.
D000818:Animals; D004247:DNA / Q000378:metabolism* / Q000528:radiation effects; D011564:Furocoumarins / Q000494:pharmacology*; D006168:Guinea Pigs; D008730:Methoxsalen / Q000494:pharmacology; D008854:Microscopy, Electron; D009691:Nucleic Acid Denaturation; D009695:Nucleic Acid Renaturation; D010777:Photochemistry; D010778:Photochemotherapy; D012867:Skin / Q000378:metabolism*; D013329:Structure-Activity Relationship; D014466:Ultraviolet Rays
D011564:Furocoumarins; D004247:DNA; D008730:Methoxsalen