Inhibition of histidine ammonia lyase by 8-methoxypsoralen and psoralen-oxidized photoproducts.

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.