The photodecomposition of aminopterin

Aminopterin is used in cell fusion experiments to select for hybrid cells by killing unfused cells which are deficient in enzymes for nucleotide salvage pathways. Aqueous aminopterin solutions exposed to fluorescent room light (wavelength greater than 300 nm) were found to lose cytotoxicity. Inactivation was accompanied by changes in the ultraviolet absorption spectra of the solutions. The ultraviolet spectrum of an irradiated aminopterin solution is used to provide a quantitative measure of its cytotoxicity. Aminopterin appears to be photolytically cleaved at the C9-N10 methylene-anilino bond in a reaction analogous to that known for the photolysis of folic acid irradiated at 365 nm (Lowry et al., 1949).