Two fluorescent heme degradation compounds are detected during autoxidation of oxyhemoglobin. These fluorescent compounds are similar to fluorescent compounds formed when hydrogen peroxide reacts with hemoglobin. Low levels of heme degradation in the presence of superoxide and catalase are attributed to a reaction involving the superoxide produced during autoxidation. The inhibition of most of the degradation by catalase suggests that the hydrogen peroxide generated during autoxidation of oxyhemoglobin produces heme degradation by the same mechanism as the direct addition of hydrogen peroxide to hemoglobin. The formation of the fluorescent degradation products was inhibited by the peroxidase substrate, ABTS, which reduces ferrylhemoglobin to methemoglobin, indicating that ferrylhemoglobin is produced during the autoxidation of hemoglobin. It is the transient formation of this highly reactive Fe(IV) hemoglobin, which is responsible for most of the heme degradation during autoxidation. (C) 2000 Academic Press.