Defense of Cutaneous Cells Against Uv Irradiation. II. Restricted Photomediated Binding of Trimethyl Psoralen to Pigmented Skin in Vivo

Distinct differences in grain distribution were seen in autoradiographs of pigmented and nonpigmented skin from spotted guinea-pig ears subjected to photosensitizing conditions by the topical application of tritiated trimethyl psoralen (3H TMP) and exposure to UV irradiation at 365 nm. Grains were localized over cells throughout the epidermis in non pigmented skin, but they were not seen beneath the stratum corneum in pigmented skin. Since TMP covalently binds to epidermal nucleic acids only after irradiation, these experiments demonstrate that the compacted melanized cells of the outer epidermis shield the DNA of underlying cells by severely restricting the penetration of UV light into the skin