Effect of Ultraviolet Irradiation on Biosynthesis of Dna in Guinea-Pig Skin in Vivo

The molecular and metabolic alterations preceding the clinical manifestation of a photobiologic process, the erythematous or sunburn reaction, were investigated in mammalian skin in vivo. The effect of a moderate (2.5–3 times the minimal erythema dose [MED]) and a large (6–8 times MED) dose of ultraviolet radiation (290–320 nm) on the incorporation of [Me-3H]-thymidine into epidermal cell DNA of guinea pigs was studied. The epilated half of the back of each animal was irradiated with various doses of ultraviolet light, and the other half served as the nonirradiated control. The amount of intraperitoneally injected [Me-3H]-thymidine incorporated into the DNA was determined by the isolation of DNA at various time intervals and the measurement of its radioactivity. Significant inhibition of the biosynthesis of DNA (64% of the control) immediately after ultraviolet irradiation was observed. The duration of this inhibition is related to the total dose of ultraviolet light delivered and to the degree of the erythema reaction (i.e., the degree of cellular damage) and can persist from 2.24hr. With a moderate dose, the biosynthesis of new DNA occurs 2hr after irradiation.