Oxidation of indole by cytochrome P450 enzymes

Indole is a product of tryptophan catabolism by gut bacteria and is absorbe d into the body in substantial amounts. The compound is known to be oxidize d to indoxyl and excreted in urine as indoxyl (3-hydroxyindole) sulfate. Fu rther oxidation and dimerization of indoxyl leads to the formation of indig oid pigments. We report the definitive identification of the pigments indig o and indirubin as products of human cytochrome P450 (P450)-catalyzed metab olism of indole by visible, H-1 NMR, and mass spectrometry. P450 2A6 was mo st active in the formation of these two pigments, followed by P450s 2C19 an d 2E1. Additional products of indole metabolism were characterized by HPLC/ UV and mass spectrometry. Indoxyl (3-hydroxyindole) was observed as a trans ient product of P450 2A6-mediated metabolism; isatin, 6-hydroxyindole, and dioxindole accumulated at low levels. Oxindole was the predominant product formed by P450s 2A6, 2E1, and 2C19 and was not transformed further. A stabl e end product was assigned the structure 6H-oxazolo[3,2-n:4,5-b']diindole b y UV, H-1 NMR, and mass spectrometry, and we conclude that P450s can cataly ze the oxidative coupling of indoles to form this dimeric conjugate, On the basis of these results, we propose that the P450/NADPH-P450 reductase syst em can catalyze oxidation of indole to a variety of products.