Ornithine decarboxylase induction in psoralen-treated mouse epidermis used as a test of UV-A sunscreen potency

Inhibition by sunscreens of UV-A-induced epidermal ornithine decarboxylase (ODC) activity in 8-methoxypsoralen-treated mice was used to examine the UV-A sunscreen activity of two commercial preparations, Uval® (para-aminobenzoic acid) and PreSun® (a benzophenone derivative). Both gave significant protection against UV-A doses of up to 3 J/cm² and Uval® was more effective that PreSun® in the UV-A range. At intermediate doses (5–15 J/cm²) neither Uval® nor PreSun® significantly altered the ODC response to UV-A. At doses of 20–40 J/cm² UV-A, control and PreSun®-treated animals showed a decline in ODC activity which histological studies suggested was due to severe epidermal damage. This decline in ODC activity and severe epidermal damage was not seen in Uval® -treated animals. Topical zinc oxide was highly effective in preventing induction of ODC by UV-A irradiation at doses up to 40 J/cm², and was significantly more effective than either PreSun® or Uval®.