H2O2-mediated cross-linking between lactoperoxidase and myoglobin -: Elucidation of protein-protein radical transfer reactions

The H2O2-dependent reaction of lactoperoxidase (LPO) with sperm whale myoglobin (SwMb) or horse myoglobin (HoMb) produces LPO-Mb cross linked species, in addition to LPO and SwMb homodimers. The HoMb products are a LPO(HoMb) dimer and LPO(HoMb), trimer. Dityrosine cross-links are shown by their fluorescence to be present in the oligomeric products. Addition of H2O2 to myoglobin (Mb), followed by catalase to quench excess H2O2 before the addition of LPO, still yields LPO cross-linked products. LPO oligomerization therefore requires radical transfer from Rib to LPO. In contrast to native LPO, recombinant LPO undergoes little self-dimerization in the absence of Mb but occurs normally in its presence. Simultaneous addition of 3,5-dibromo-4-nitrosobenzenesulfonic acid (DBNBS) and LPO to activated Mb produces a spin-trapped radical electron paramagnetic resonance signal located primarily on LPO, confirming the radical transfer. Mutation of Tyr-103 or Tyr-151 in SwMb decreased cross-linking with LPO, but mutation of Tyr-146, Trp-7, or Trp-14 did not. However, because DBNBS-trapped LPO radicals were observed with all the mutants, DBNBS traps LPO radicals other than those involved in protein oligomerization. The results clearly establish that radical transfer occurs from Mb to LPO and suggest that intermolecularly transferred radicals may reside on residues other than those that are generated by intramolecular reactions.