Identification, Occurrence, and Mechanism of Formation of 1-Acetyl-β-carbolines Derived from l-Tryptophan and Methylglyoxal.

β-Carbolines (βCs) are bioactive compounds present in foods and biological systems. This work describes the identification, occurrence, and mechanism of formation of 1-acetyl-β-carbolines (1-acetyl-βCs) that result from the reaction of l-tryptophan with the α-dicarbonyl compound methylglyoxal. Two β-carbolines are characterized as 1-acetyl-β-carboline (AβC) and 1-acetyl-β-carboline-3-carboxylic acid (AβC-COOH). Their formation was favored in acidic conditions and with increasing temperature, but 1-acetyl-βCs also formed in moderate temperatures and in a wide range of pH, including physiological conditions, and in human serum. The formation mechanism relies on tautomerism and cyclization to give 1-(1-hydroxyethyl)-3,4-dihydro-β-carboline-3-carboxylic acid intermediates followed by the oxidation of C'-OH and aromatization to 1-acetyl-βCs. The formation of 1-acetyl-βCs took place in the reactions of fructose or glucose with tryptophan under heating and depended on the methylglyoxal released during carbohydrate degradation. Formation from carbohydrates increased at neutral or basic pH values as more methylglyoxal was released under those conditions. Thus, 1-acetyl-βCs could be advanced glycation end-products (AGEs). 1-Acetyl-βCs were identified and quantified for the first time in many foods. AβC ranged from undetectable to 250 ng/g with the highest amount detected in honey, bread, cookies, soy sauce, and coffee. On average, AβC-COOH generally appeared in lower concentrations than AβC but it ranged from undetectable to 323 ng/g with the highest levels found in soy sauce, honey, cookies, and fried bread. These results indicate that 1-acetyl-βCs could be relevant βCs in foods and in vivo.