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Ultraviolet B (UVB, 290-320 nm) radiation initiates in vivo a dose- and wavelength-dependent down regulation of cell-mediated immunity. An action spectrum for UV-induced immunosuppression indicated that the photoreceptor for this effect is urocanic acid (UCA), which undergoes a trans to cis isomerization in the stratum corneum on UV exposure. An accumulation of evidence has supported this conclusion. However, evidence has also been presented that formation of thymine dimers in DNA is responsible for initiation of UV-induced immunosuppression. Because photobinding of UCA to DNA in vitro forming cyclobutane-type adducts has been shown, we sought to resolve this dilemma by investigating if UCA photobinds to DNA in vivo. The [14C]cis-UCA, [14C]trans-UCA or [3H]8-MOP (8-methoxypsoralen) was applied topically to BALB/c mice that were then irradiated with a dose of UV previously shown to cause systemic suppression of contact hypersensitivity. The DNA was prepared from epidermal cells by phenol extraction immediately after in vivo irradiation and bound radioactivity determined. Although photobinding of [3H]8-MOP was readily demonstrable under these conditions (0.9 nmol/mg DNA), no significant binding of either isomer of UCA to DNA (between 1.2 x 10(-3) and 2.1 x 10(-3) ng/mg DNA) could be detected. Uptake studies in keratinocytes prepared from epidermis of untreated animals indicated that [3H]8-MOP was taken up with a rate constant of 4.2 x 10(-3) pmol/s/mg protein/mumol/L. In contrast, uptake of [14C]cis-UCA was not statistically significant from zero and uptake of [14C]trans-UCA was negligible (0.8 x 10(-3) +/- 0.08 x 10(-3) pmol/s/mg protein/mumol/L). There was no significant difference between uptake of UCA isomers, but uptake of [3H]8-MOP was significantly greater than that of either UCA isomer (P0.01). These studies indicate that the photobinding of UCA to DNA does not play a role in UV-induced immunosuppression.