2-Aminoadipic acid is a marker of protein carbonyl oxidation in the aging human skin: effects of diabetes, renal failure and sepsis

We hypothesized that the epsilon-amino group of lysine residues in long-lived proteins oxidatively deaminates with age forming the carbonyl compound, allysine (alpha-aminoadipic acid-delta-semialdehyde), which can further oxidize into 2-aminoadipic acid. In the present study, we measured both products in insoluble human skin collagen from n = 117 individuals of age range 10-90 years, of which n = 61 and n = 56 were non-diabetic and diabetic respectively, and a total of n = 61 individuals had either acute or chronic renal failure. Allysine was reduced by borohydride into 6-hydroxy-norleucine and both products were measured in acid hydrolysates by selective ion monitoring gas chromatography (GC)-MS. The results showed that 2-aminoadipic acid (P < 0.0001), but not 6-hydroxynorleucine (P = 0.14), significantly increased with age reaching levels of 1 and 0.3 mmol/mol lysine at late age respectively. Diabetes in the absence of renal failure significantly (P < 0.0001) increased 2-aminoadipic acid up to < 3 mmol/mol, but not 6-hydroxynorleucine (levels < 0.4 mmol/mol, P = 0.18). Renal failure even in the absence of diabetes markedly increased levels reaching up to < 0.5 and 8 mmol/mol for 6-hydroxynorleucine and 2-aminoadipic acid respectively. Septicaemia significantly (P < 0.0001) elevated 2-aminoadipic acid in nondiabetic, but not diabetic individuals, and mildly correlated with other glycoxidation markers, carboxymethyl-lysine and the methylglyoxal-derived products, carboxyethyl-lysine, argpyrimidine and MODIC (methylglyoxal-derived imidazolium crosslink). These results provide support for the presence of metal-catalysed oxidation (the Suyama pathway) in diabetes and the possible activation of myeloperoxidase during sepsis. We conclude that 2-aminoadipic acid is a more reliable marker for protein oxidation than its precursor, allysine. Its mechanism of formation in each of these conditions needs to be elucidated.