Individual modification site preferences of α-lactalbumin glycated in the dry state by glucose, maltotriose and galacturonic acid

The differences in maximum Maillard glycation extent of proteins incubated with saccharides of different size/charge might be explained by differences in saccharide reactivity towards each protein modification site. Mixtures of α-lactalbumin-glucose (AG), α-lactalbumin-maltotriose (AMTT) or α-lactalbumin-galacturonic acid (AGalA) were incubated for 10 h, reaching a glycation extent of 42, 19 and 30 %, respectively. Glycated protein samples were enzymatically hydrolyzed followed by UPLC-UV-MS determination and quantification of glycation in individual modification sites. On average, around 80 % of protein glycation was recovered as glycated peptides. For all protein-saccharide mixtures, the N-terminal amino group, K58, K62 and K114 were never glycated, whilst K5 and K122 were always the most glycated sites. The glycated sites R10, K79, K93, K94 and K108 found in AGalA, were absent from AG and AMtt. Differences in glycated/non-glycated sites could not explain the differences in total protein glycation between the various samples.