Identification of glycation at the N-terminus of albumin by gas chromatography-mass spectrometry

Amino groups in human albumin are modified in vivo by glucose in a non-enzymic reaction, and previous studies have implicated lysine residues as exclusive participants. An investigation using g.c.-m.s. was undertaken to ascertain whether or not the N-terminus was also involved. Appropriate model compounds [N-(1-deoxyglucitol-1-yl) and N-(1-deoxymannitol-1-yl) adducts of aspartic acid] were synthesized and the diagnostic fragment ions of suitable derivatives were established under electron-impact and negative-chemical-ionization conditions. Characteristic fragment ions were identical with those obtained from the model compounds in the mass spectra of derivatives prepared from hydrolysates of reduced albumin. A purified mixture of the model compounds was also obtained from such hydrolysates. Use of radioisotopic incorporation demonstrated that the relative extent of glycation of the epsilon-amino and alpha-amino groups in albumin was approx. 8:1. N-1-Deoxyhexitol adducts of aspartic acid were also identified in reduced and hydrolysed peptides of human urine.