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Myoglobin (Mb) redox state affects meat color and is destabilized by lipid oxidation products such as 4-hydroxy-2-nonenal (HNE). Our objective was to investigate lipid oxidation-induced oxymyoglobin (OxyMb) oxidation in Mb from two major meat-producing livestock species utilizing MS and proteomics tools. Porcine OxyMb was incubated with HNE and analyzed for metmyoglobin (MetMb) formation. MetMb formation was greater in the presence of HNE than controls at pH 7.4 and 37 degrees C (p <0.05). MALDI-TOF MS was used to identify adduct formation; only mono-adducts of HNE (via Michael addition) with porcine Mb were detected. LC-ESI-MS/MS identified three histidine (HIS) residues in porcine Mb that were readily adducted by HNE (HIS 24, 36 and 119), whereas in bovine Mb seven histidine residues (HIS 24, 36, 81, 88, 93, 119 and 152) were adducted. Quantitation of HNE-adducted peptides using isotope-labeled phenyl isocyanate indicated that, initially, HIS 36 was preferentially adducted in porcine Mb whereas HIS 81, 88 and 93 were the predominant sites of early HNE adduction in bovine Mb. Preferential HNE adduction at the proximal histidine (HIS 93) was observed exclusively in bovine OxyMb and may explain why lipid oxidation-induced OxyMb oxidation appears more extensive in beef, than in pork.