Anthralin inhibition of mouse epidermal arachidonic acid lipoxygenase in vitro

Epidermal strips, free of sebaceous gland and hair follicle contamination, were prepared from mouse tail skin. Epidermal homogenates synthesized prostaglandins and 12-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE) from exogenously added [1-14C]arachidonic acid. The effects of pH, assay time, substrate concentration, and several selective inhibitors upon the lipoxygenase and cyclooxygenase pathways were determined. Ultracentrifugation of the crude homogenate at 105,000 g sedimented both activities, and pellet 12-HETE synthesis increased 2-fold relative to the crude homogenate. Recombination of the 105,000 g pellet and supernatant gave yields of prostaglandins and 12-HETE essentially equivalent to that of crude homogenate. When tested in homogenate with 4.5 microM arachidonic acid, anthralin specifically inhibited 12-HETE production with IC50 of 50.0 microM; no significant effect against cyclooxygenase was observed over the dose range of 2-200 microM. 1,8-Dihydroxy-9,10-anthraquinone (DHAQ) also specifically inhibited 12-HETE synthesis, but the dose response curve was flatter and maximum inhibition was only 55% at 200 microM. 6-Chloro-2,3-dihydroxy-1,4-naphthoquinone (CDNQ), an agent with topical antipsoriatic activity, also inhibited 12-HETE synthesis with an IC50 of 25 microM, but simultaneously stimulated prostaglandin production, up to 2.5-fold at 200 microM. When tested with washed human platelets, anthralin again specifically inhibited 12-HETE production with an IC50 of 10 microM, while DHAQ inhibited lipoxygenase activity by only 40% at 25 microM. When tested in platelets, CDNQ gave 33% inhibition of 12-HETE production at 200 microM, although prostaglandin synthesis was stimulated over the range of 25-200 microM. It is proposed that certain antipsoriatic agents may exert their action through modulation of arachidonic acid metabolism.