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.

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.