beta-Carbolines are naturally occurring bioactive alkaloids. In this work, carbohydrate-derived beta-carbolines (beta Cs), 1-(1,3,4,5-tetrahydroxypent-1-yl)-beta-carboline isomers (1a/b), 1-(1,4,5-trihydroxypent-1-yl)-beta-carboline (2), 1-(1,5-dihydroxypent-3-en-1-yl)-beta-carboline (3), and 1-(1,2,3,4,5-pentahydroxypent-1-yl)-beta-carboline (4) were identified and analyzed in commercial foods. The concentrations of beta Cs 1-4 in foods ranged from undetectable to 11.4 mu g/g levels, suggesting their intake in the diet. Processed foods contained higher amounts than fresh or unprocessed foods, and the highest content was found in processed tomato and fruit products, sauces, and baked foods. beta Cs 1-3 were formed in foods during heating, and 1a/b were the main compounds. The formation of carbohydrate-derived beta Cs was studied in model reactions of tryptophan and carbohydrates. They formed in reactions of tryptophan with glucose under acidic conditions at temperatures higher than 80 degrees C. The formation of 1a/b was favored, but 2-3 increased at high temperatures. Noticeably, the beta Cs 1-3 formed in the reactions of tryptophan with fructose or sucrose, and the formation from fructose was much higher than from glucose. Thus, fructose was the main carbohydrate involved in the formation of 1-3, whereas sucrose gave these beta Cs after acid hydrolysis. It is shown for the first time that the mechanism of formation of beta Cs 1- 3 occurs from the sugar intermediate 3-deoxyglucosone that reacts with tryptophan affording these carbohydrate-derived beta Cs. A mechanism of reaction to give beta Cs 1-3 is proposed that relies on the tautomerism (keto-enediol or enamine-imine) of intermediates involved in the reaction. Carbohydrate beta Cs 1-4 were assessed as inhibitors of monoamine oxidase (MAO), as antioxidants, and for their interaction with DNA. They were not good inhibitors of MAO-A or -B, were poor antioxidants, and did not appreciably interact with DNA.