Metal-catalyzed oxidation of brain-derived neurotrophic factor (BDNF):: Analytical challenges for the identification of modified sites

Purpose. We examined the metal-catalyzed oxidation of brain-derived neurotrophic factor (BDNF) using the Cu(II)ascorbate/O-2 model oxidative system. Methods. Electrospray ionization mass spectrometry, peptide mapping and amino acid analysis were utilized to determine the nature of the covalent modification induced by the metal-catalyzed oxidative system. Additionally, analytical ultracentrifugation, the Bradford assay, circular dichroism and ANSA dye-binding were used to determine the nature of any conformational changes induced by the oxidation. Results. Exposure of BDNF to the Cu(II)/ascorbate/O-2 system led to the modification of ca. 35% of Met(92) to its sulfoxide, and to subsequent conformational changes. The proteolytic digestion procedure was sensitive to this conformational change, and was unable to detect the modification. Chemical digestion with CNBr, however, was not sensitive to this change. and allowed for the identification of the site of modification. Conclusions. The modification of Met(92) to its sulfoxide rendered the oxidized BDNF inaccessible to proteolytic digestion, due to conformational changes associated with the oxidation.