Lactolation of β-lactoglobulin monitored by electrospray ionisation mass spectrometry

Direct monitoring of the Amadori product formation between lactose and beta-lactoglobulin was performed by electrospray ionisation mass spectrometry. As expected, the glycation reaction was faster at low water activity than in aqueous system. Heterogeneous beta-lactoglobulin glycoforms were observed, with different number of lactose bound: populations of beta-lactoglobulin with 1 to 7 and 2 to 11 linked lactose were detected, respectively, after 10 and 22 h of reaction under 65% relative humidity and 50 degrees C. Trypsinolysis of glycated proteins, followed by reverse-phase HPLC coupled to tandem mass spectrometry in the neutral loss scanning mode, indicated that all potential reactive amino groups, except Lys(101), were involved in sugar binding. The resulting order of reactivity, as a function of reaction time, was as follows: Lys(74,91) > alpha-amino-group, Lys(15,70,100) > Lys(60,69,75,77,83,135,138) > Lys(8,141). (C) 1998 Elsevier Science Ltd. All rights reserved.