Abstract
The effect of 8‐methoxypsoralen‐UVA therapy on the catalysis of histidine to trans‐urocanic acid by histidine ammonia lyase (HAL, EC 4.3.1.3) was examined using an enzymatic assay from Sigma‐Aldrich where the growth of the trans‐urocanic acid peak at 277 nm was monitored. A Rayonet Photochemical Mini Reactor (Model RMR‐600) equipped with eight, 3500 Å light sources and a custom UVA filter (Model S‐BAL3 2.9 mm), from the Solar Light Company, were used to expose various reaction mixtures to broadband UVA light and UVA/UVB light. A UV‐Vis spectrophotometer (Model Shimadzu UV 2540) with a temperature‐controlled cell holder (Model TCC240) was used to monitor the growth of the trans‐urocanic peak. Results of dark‐binding experiments of 8‐methoxypsoralen in denatured ethanol indicate no inhibition of enzyme activity due to ethanol but noncompetitive inhibition due to 8‐methoxypsoralen. The effects of preirradiated 8‐methoxypsoralen, with both broadband UVA and UVA/UVB, indicate that inhibition was due to psoralen‐oxidized photoproducts. Inhibition of HAL was found when exposed to broadband UVA/UVB and to a lesser extent when exposed to broadband UVA.