The role of methylglyoxal in the non-enzymatic conversion of tryptophan, its methyl ester and tryptamine to 1-acetyl-β-carbolines

Non-enzymatic modification of l-tryptophan (1) and its metabolites and derivatives with aldehydes, via the Pictet–Spengler reaction, affords β-carbolines. Here we demonstrate that methylglyoxal (2) generates 1-acetyl-β-carbolines from tryptophan (1), from its methyl ester (6) and from tryptamine (4); however, 2 did not generate 1-(1-hydroxyethyl)-β-carboline derivates. HPLC analysis of model reaction systems showed formation of 1-acetyl-β-carboline (3) and 1-acetyl-β-carboline-3-carboxylic acid (5) during incubation of 1 and 2, at pH 5.7 and 7.4, at 100 °C, and only 5 at 37 °C, at the same pH values, with limited access of oxygen. Aerobic conditions caused higher formation of 3 at 37 °C at both pH values, while, at higher temperature, the same effect was only observed at pH 5.7. Lack of oxygen did not much influence the formation of 3 or 5 at both pH and temperature values, in comparison with the formation at limited access of oxygen. Incubation of 2 and 6 generated methyl-1-acetyl-β-carboline-3-carboxylate (7) together with 3 and 5 as a result of hydrolysis of 6 into 1 and, partially, 7 into 5, while in incubation mixtures of 2 and 4 only unstable 1-acetyl-3,4-dihydro-β-carboline (8) was observed. Incubation of 1 with d-glucose as well as incubation of tryptophan with Amadori product 18 under similar conditions did not generate carbolines 3 or 5. For the first time, we were able to demonstrate the presence of 1-acetyl-β-carboline-3-carboxylic acid (5) in some commercially available ketchups and in previously heated tomato concentrate.

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