DETECTION OF LOW-LEVEL LIGHT-EMISSION AND FORMATION OF TOXIC RADICAL SPECIES DURING METHEMOGLOBIN AND METMYOGLOBIN FORMATION

Many environmental pollutants (e.g., nitrite), pharmaceuticals (e.g., hydroxyurea) and food additives (e.g., butylated hydroxyanisole) play a significant role in the formation of methemoglobin from hemoglobin, which loses its oxygen binding properties. All these substances are re ductants which are oxidized by the heme-bound dioxygen thereby generat ing free radicals which play an important pathogenetic role. In additi on to the acute effects (methemoglobin formation, anemia), which have already been investigated thoroughly, chronic effects are expected if the body is exposed to these chemicals over an extended period of time . As representatives of the heme enzymes the reaction of bovine hemogl obin and equine myoglobin with reducing xenobiotics such as hydroxylam ines or phenols were investigated. The reaction mechanism consists of several consecutive steps, the first step being the one-electron oxida tion of the xenobiotic to the respective free radical. According to th e reaction stoichiometry, the heme-bound dioxygen is reduced to the ox idation state (-I) of hydrogen peroxide, although no release of free h ydrogen peroxide into the solution was detected. Instead, we observed the formation of a compound l-type ferrylheme species, which was accom panied by low level chemiluminescence. The same complex was formed in a model system consisting of methemoglobin or metmyoglobin and hydroge n peroxide. A prerequisite for the formation of this compound I specie s was the presence of a globin moiety together with a free sixth ligan d position on the heme moiety. A tentative reaction scheme for the lig ht emitting step was postulated which involved the transition of the h eme iron from Fe(+III) to Fe (+IV) and the concomitant formation of a globin-bound protein radical (Tyr-O. or Cys-S.). All reaction intermed iates are highly reactive prooxidants of toxicological interest.