Photochemical oxygen consumption, oxygen evolution and spectral changes during UVA irradiation of EMT6 spheroids

Remarkable rates of oxygen consumption are observed via microelectrode meas urements immediately upon the onset of 325 nm irradiation of multicell tumo r spheroids, Consumption is irradiance dependent over the range 20-200 mW c m(-2), and its magnitude is comparable to that observed previously in the s ame system using exogenous photosensitizers. Oscillations in the oxygen con centrations suggest that oxygen is also being evolved during irradiation. O xygen evolution is likely the result of enzymatic dissociation of hydrogen peroxide, which is formed through UV-induced photochemistry. Irradiation of spheroids at 442 and at 514 nm produces a much more modest but detectable oxygen consumption. The dynamics of oxygen concentration changes are quite different at these wavelengths, suggesting a different photochemical mechan ism. In these cases, initial oxygen depletion is followed immediately by a more gradual, monotonic increase in the oxygen concentration, consistent wi th irreversible photobleaching. No oscillations in the oxygen concentration are detectable. At 662 nm, no oxygen consumption was observed over the ran ge of irradiances studied. Fluorescence spectra of cells prior to irradiati on include contributions from anthranilic acid and reduced nicotinamide ade nine dinucleotide (NADH). During 325 nm irradiation, anthranilic acid is ra pidly and irreversibly bleached, while NADH emission undergoes only modest reduction.