REACTION OF FERRIC LEGHEMOGLOBIN WITH H2O2 - FORMATION OF HEME-PROTEIN CROSS-LINKS AND DIMERIC SPECIES

Ferric leghemoglobin in the presence of H2O2 is known to give rise to protein radicals, at least one of which is centred on a tyrosine resid ue, These radicals are quenched by at least two processes. The first o ne involves an intramolecular heme-protein cross-link probably involvi ng the tyrosine radical; this leads to the formation of a green compou nd with spectral characteristics differing markedly from those of ferr yl and ferric leghemoglobin. This green compound cannot be reduced by dithionite or ascorbate, precluding any role for this species as an ox ygen carrier, It exhibits modified EPR and pyridine haemochromogen spe ctra, indicating that alterations occur at the porphyrin macrocycle le vel, The additional compound previously described [Puppo, A., Monny, C . and Davies, M.J. (1993) Biochem. J. 289, 435-438] appears to be a mi xture of ferryl Lb and this green compound. The second quenching route results in the formation of intermolecular cross-links and hence dime ric forms of the protein. Ascorbate and glutathione inhibit both this intermolecular dimer formation and the formation of the intramolecular haem-protein cross-links and are likely to play a protective role in vivo.