Naturally-occurring tetrahydro-β-carboline alkaloids derived from tryptophan are oxidized to bioactive β-carboline alkaloids by heme peroxidases

•Bioactive β-carbolines are indole alkaloids occurring in plants and mammals.•β-Carbolines arise from tetrahydro-β-carbolines by unknown way and mechanism.•Heme peroxidases oxidized tetrahydro-β-carbolines to β-carbolines.•Oxidation followed a typical catalytic cycle of heme peroxidases.•Mammalian (MPO and LPO) and plant (HRP) heme peroxidases catalyzed oxidation. β-Carbolines are indole alkaloids that occur in plants, foods, and endogenously in mammals and humans, and which exhibit potent biological, psychopharmacological and toxicological activities. They form from naturally-occurring tetrahydro-β-carboline alkaloids arising from tryptophan by still unknown way and mechanism. Results in this research show that heme peroxidases catalyzed the oxidation of tetrahydro-β-carbolines (i.e. 1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid and 1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid) into aromatic β-carbolines (i.e. norharman and harman, respectively). This oxidation followed a typical catalytic cycle of peroxidases through redox intermediates I, II, and ferric enzyme. Both, plant peroxidases (horseradish peroxidase, HRP) and mammalian peroxidases (myeloperoxidase, MPO and lactoperoxidase, LPO) catalyzed the oxidation in an efficient manner as determined by kinetic parameters (VMAX and KM). Oxidation of tetrahydro-β-carbolines was inhibited by peroxidase inhibitors such as sodium azide, ascorbic acid, hydroxylamine and excess of H2O2. The formation of aromatic β-carbolines by heme peroxidases can help to explain the presence and activity of these compounds in biological systems.