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Temperature dependence of hemolysin production during psoralen photooxidation Vladislava O. Melnikovaa,b, Mikhail V. Malakhova, Elena S. Andinaa, Eugene P. Lysenkoa, Irina V. Belichenkoa,b, Lina N. Bezdetnayab, Francois Guilleminb and Alexander Ya. Potapenkoa*. aDepartment of Medical and Biological Physics, Russian State Medical University, Moscow, Russia bUnite de Recherche en Therapie Photodynamique, Centre Alexis Vautrin, Vandoeuvre-les-Nancy, France Abstract There are indications that toxic and therapeutic effects induced by PUVA-therapy (psoralen+UVA) are at least partly mediated by products of psoralen photooxidation (POP). In the present study psoralen photooxidation in aqueous solution was investigated in temperature range from 4 to 45° C. Chemical identification of biologically active POP products is highly difficult because of their instability. That is why POP products were revealed with the use of very sensitive biologic test system - by their hemolytic effects in suspensions of human erythrocytes. The production of POP hemolysins (POPhem) increased with temperature in the range from 4 to 25° C. At temperature higher than 25° C, the production of POPhem dropped down. Analogous temperature dependence was observed when POP products were estimated by chemiluminescence induced by addition of ferrous ions to aqueous POP solution (POPChL). However, despite the resemblance in temperature dependencies of POP production revealed by hemolysis and chemiluminescence, these two methods gave information about different products of psoralen photooxidation because POPChL were more labile in storage than POPhem. Therefore, estimation of products of psoralen photooxidation by their hemolytic effect is a unique and specific approach for monitoring the production and properties of biologically active products of photooxidation. *Corresponding author: Department of Medical and Biological Physics, Russian State Medical University, Ostrovityanova Street 1, Moscow 117869, Russia. Fax: +007-095-2451220; e-mail: potap@hotmail.com