Biological effects of menadione photochemistry: effects of menadione on biological systems may not involve classical oxidant production

Because cell-mediated reduction of menadione leads to the generation of rea ctive oxygen species (ROS), this quinone is widely used to investigate the effects of ROS on cellular functions. We report that A549 human lung epithe lial cells exposed to menadione demonstrate a dose-dependent increase in bo th intracellular calcium ([Ca2+](i)) and ROS formation. The concentrations of menadione required to initiate these two events are markedly different, with ROS detection requiring higher levels of menadione. Modulators of anti oxidant defences (e.g. buthionine sulphoximine, 3-amino-1,2,4-triazole) hav e little effect on the [Ca2+](i) response to menadione, suggesting that ROS formation does not account for menadione-dependent alterations in [Ca2+](i ). Additional evidence suggests that menadione photochemistry may be respon sible for the observed [Ca2+](i) effects. Specifically: (a) EPR studies wit h the spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) show that light exp osure (maximum effect at 340 nm) stimulates menadione-dependent formation o f the DMPO/(OH)-O-. spin adduct that was not sensitive to antioxidant inter ventions; (b) DMPO inhibits menadione and light-dependent increases in [Ca2 +](i); and (c) light (maximum effect at 340 nm) augments the deleterious ef fects of menadione on cell viability as determined by Cr-51 release. These photo effects do not appear to involve formation of singlet oxygen by menad ione, but rather are the result of the oxidizing chemistry initiated by men adione in the triplet state. This work demonstrates that menadione species generated by photo-irradiation can exert biological effects on cellular fun ctions and points to the potential importance of photochemistry in studies of menadione-mediated cell damage.