Table 1.

Mass spectrometry analysis of tryptic MPO peptides indicated three peptides had an increased mass corresponding to the formation of the benzoic acid adducts. Comparison was made between peptides observed in the MPO protein alone and a reaction of MPO (1.2 µM) with BAH (5 mM) in the presence of H₂O₂ (40 µM) in 50mM sodium acetate buffer at pH 5.7. The statistical probabilities for the correct identification peptides are shown by the peptide–spectrum matches with the Sequest score (XCorr) and the posterior error probability (PEP) values. The smaller the PEP value, the more certainty there is the identification of a correctly matched peptide.

MPO amino acid	Identified modified peptide	Oxidized residue	MH+ [Da]	XCorr	PEP
HCLys138	IkNQADcIPFFR	M2	1612.79	2.25	7.86E-3
HCLys308	kYLPTYR	M1	1044.55	2.18	5.9E-2
HCLys463	kLMEQYGTPNNIDIWMGGVSEPLKR	M1	2980.47	4.13	6.4E-5