RADIATION-CHEMISTRY OF PHYSIOLOGICAL SALINE REINVESTIGATED - EVIDENCETHAT CHLORIDE-DERIVED INTERMEDIATES PLAY A KEY ROLE IN CYTOTOXICITY

Contrary to common belief, hydrogen peroxide (H2O2) and hypochlorite ( HOCl) are not produced continuously and independently during the irrad iation of buffer solution containing chloride. Different buildup and d ecay reactions are involved in a complex interaction of these substanc es during irradiation. Which of the species predominates is determined by the parameters of the solution. The amount of either compound dete ctable after irradiation depends on the dissolved gas (O-2, N2O or N-2 ), on the pH value and to some extent on the presence of catalytic met als: Under slightly acidic conditions, low oxygen content and high gen eration rates of OH radicals, the only detectable species is hypochlor ite; at high oxygen content and at pH values in the physiological rang e, hydrogen peroxide is the main detectable product. However, H2O2 and HOCl react with each other in a pH-dependent way, yielding the stable products O-2 and Cl-. This reaction limits the expected lifetime of b oth species in aqueous solution to some tens of seconds, Therefore, an alysis of the sample solution after irradiation determines only the su bstance that was present in greater relative concentration at the term ination of irradiation. Such analysis, however, does not allow conclus ions about the processes that occurred during irradiation. We have inv estigated the decay and formation reactions of H2O2 and HOCl under all relevant irradiation conditions and found evidence that the formation and further reaction of HOCl.-, the precursor of HOCl, is of central importance even in cases where no significant amounts of H2O2 or HOCl are detectable after irradiation. We discuss the consequences of these results for the cytotoxicity observed after irradiation of cells susp ended in physiological saline and conclude that analogous processes mu st also be relevant for irradiations under in vivo conditions. (C) 199 7 by Radiation Research Society