Deconstruction of activity-dependent covalent modification of heme in human neutrophil myeloperoxidase by multistage mass spectrometry (MS(4)).

Myeloperoxidase (MPO) is known to be inactivated and covalently modified by treatment with hydrogen peroxide and agents similar to 3-(2-ethoxypropyl)-2-thioxo-2,3-dihydro-1H-purin-6(9H)-one (1), a 254.08 Da derivative of 2-thioxanthine. Peptide mapping by liquid chromatography and mass spectrometry detected modification by 1 in a labile peptide-heme-peptide fragment of the enzyme, accompanied by a mass increase of 252.08 Da. The loss of two hydrogen atoms was consistent with mechanism-based oxidative coupling. Multistage mass spectrometry (MS(4)) of the modified fragment in an ion trap/Orbitrap spectrometer demonstrated that 1 was coupled directly to heme. Use of a 10 amu window delivered the full isotopic envelope of each precursor ion to collision-induced dissociation, preserving definitive isotopic profiles for iron-containing fragments through successive steps of multistage mass spectrometry. Iron isotope signatures and accurate mass measurements supported the structural assignments. Crystallographic analysis confirmed linkage between the methyl substituent of the heme pyrrole D ring and the sulfur atom of 1. The final orientation of 1 perpendicular to the plane of the heme ring suggested a mechanism consisting of two consecutive one-electron oxidations of 1 by MPO. Multistage mass spectrometry using stage-specific collision energies permits stepwise deconstruction of modifications of heme enzymes containing covalent links between the heme group and the polypeptide chain.

Modification of amyloid-beta(1-40) by a protease inhibitor creates risk of error in mass spectrometric quantitation of amyloid-beta(1-42).

Matrix-assisted laser desorption mass spectrometry successfully analyzes mixed populations of amyloid-beta (Abeta) peptides, providing a profile in which changes caused by drug action are directly observed. A spectrum of Abeta immunocaptured from guinea pig brain included a novel component with monoisotopic [M+H]+ at 4511.22, close to the monoisotopic value of [M+H]+ for Abeta(1-42) of 4512.27 and overlapping and interfering with the authentic Abeta(1-42) peak. Hypothesis and experiment led to the conclusion that modification of Abeta(1-40) by the protease inhibitor aminoethylbenzenesulfonyl fluoride generates a product with monoisotopic [M+H]+ at 4511.19, and that this accounts for the interfering peak.