UROCANIC ACID DOES NOT PHOTOBIND TO DNA IN MICE IRRADIATED WITH IMMUNOSUPPRESSIVE DOSES OF UVB

Ultraviolet B (UVB, 290-320 nm) radiation initiates ill vivo a dose-an d wavelength-dependent down regulation of cell-mediated immunity. An a ction spectrum for UV-induced immunosuppression indicated that the pho toreceptor for this effect is urocanic acid (UCA), which undergoes re trans to eis isomerization in the stratum corneum on UV exposure, An a ccumulation of evidence has supported this conclusion, However, eviden ce has also been presented that formation of thymine dimers in DNA is responsible for initiation of UV-induced immunosuppression. Because ph otobinding 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 [C-14]cis-UCA, [C-14]trans-UCA or [H-3 ]8-MOP (8-methoxypsoralen) was applied topically to BALB/c mice that w ere then irradiated with a dose of UV previously shown to cause system ic suppression of contact hypersensitivity. The DNA was prepared bean epidermal cells by phenol extraction immediately after in vivo irradia tion and bound radioactivity determined, Although photobinding of [H-3 ]8-MOP was readily demonstrable under these conditions (0.9 nmol/mg DN A), 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 studie s in keratinocytes prepared from epidermis of untreated animals indica ted that [H-3]8-MOP was taken up with a rate constant of 4.2 x 10(-3) pmol/s/mg protein/mu mol/L. In contrast, uptake of [C-14]cis-UCA was n ot statistically significant from zero and uptake of [C-14]trans-UCA w as negligible (0.8 x 10(-3) +/- 0.88 x 10(-3) pmol/s/mg protein/mu mol /L). There was no significant difference between uptake of UCA isomers , but uptake of [H-3]8-MOP was significantly greater than that of eith er UCA isomer (P < 0.01), These studies indicate that the photobinding of UCA to DNA does not play a role in UV-induced immunosuppression.