Ultra performance liquid chromatography-mass spectrometric determination of the site specificity of modification of beta-casein by glucose and methylglyoxal.

Modification of protein by carbonyl compounds under in vitro physiological conditions is site-directed. There are few reports of the site specificity of glycation of proteins using heating conditions of relevance to food processing. The aim of this study was to determine the site specificity of modification of beta-casein (betaCN) by glucose and methylglyoxal (MGO). betaCN (1.33 M, 3.2%) was heated with either glucose (1.345 M, 4.6%) or MGO (1 mM) at 95 degrees C for up to 4 h. Tryptic digests were prepared and analysed by ultra performance liquid chromatography electrospray ionisation mass spectrometry (UPLC-ES/MS). The sites of formation of the Amadori product, N(epsilon)-(fructosyl)lysine (FL), and the advanced glycation end-products, N(epsilon)-(carboxymethyl)lysine (CML), MGO-derived dihydroxyimidazolidine (MG-DH) and MGO-derived hydroimidazolone (MG-HI), were located. FL and CML were detected at K107 and K176 residues in betaCN/glucose incubations. Indigenous N ( epsilon )-(lactulosyl)lysine was detected at K107 only. MG-DH and MG-HI were detected at R202 and possibly R183 residues in both betaCN/glucose and betaCN/MGO incubations. Glycation of betaCN by glucose and MGO resulted in similar site specificity for MG-DH and MG-HI formation.