Oxidation of chlorpromazine by methemoglobin in the presence of hydrogen peroxide. Formation of chlorpromazine radical cation and its covalent binding to methemoglobin

The oxidation of chlorpromazine by methemoglobin plus H2O2 has been studied. The transient formation of the chlorpromazine radical cation in this reaction has been demonstrated by light absorption measurements. Under the experimental conditions complete conversion of chlorpromazine yields approximately 60% chlorpromazine sulfoxide. From studies with 3H-labeled chlorpromazine it appears that the remaining 40% is covalently bound to apohemoglobin. Upon reaction of methemoglobin with H2O2 a stable ferrylhemoglobin is formed. This ferrylhemoglobin is not the reactive species, which accepts the chlorpromazine electron, as its presence is not sufficient to induce chlorpromazine oxidation. For this the presence of H2O2 is a prerequisite. This indicates that a transient species in the formation of the stable ferrylhemoglobin is involved, whether this is a compound I analogue or a ferrylhemoglobin with a free radical on one of the apoprotein residues. Exposition of methemoglobin to H2O2 denatures hemoglobin and induces protein-heme crosslinks, as appears from changes in the visible absorption spectrum and heme retention by the protein after methyl ethyl ketone extraction. Reaction with CPZ partly protects against denaturation and crosslinking.