Oxidation of Indole by Cytochrome P450 Enzymes

Abstract

Indole is a product of tryptophan catabolism by gut bacteria and is absorbed into the body in substantial amounts. The compound is known to be oxidized to indoxyl and excreted in urine as indoxyl (3-hydroxyindole) sulfate. Further oxidation and dimerization of indoxyl leads to the formation of indigoid pigments. We report the definitive identification of the pigments indigo and indirubin as products of human cytochrome P450 (P450)-catalyzed metabolism of indole by visible, (1)H NMR, and mass spectrometry. P450 2A6 was most active in the formation of these two pigments, followed by P450s 2C19 and 2E1. Additional products of indole metabolism were characterized by HPLC/UV and mass spectrometry. Indoxyl (3-hydroxyindole) was observed as a transient 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 stable end product was assigned the structure 6H-oxazolo[3,2-a:4, 5-b']diindole by UV, (1)H NMR, and mass spectrometry, and we conclude that P450s can catalyze the oxidative coupling of indoles to form this dimeric conjugate. On the basis of these results, we propose that the P450/NADPH-P450 reductase system can catalyze oxidation of indole to a variety of products.

Hydroxylation of Indole by Laboratory-evolved 2-Hydroxybiphenyl 3-Monooxygenase

“…Hydroxylation of Indole by HbpA ind -Indole is oxidized by different oxygenases that contain either protein-bound iron or cytochrome to activate molecular oxygen (6,32). No flavoprotein oxygenase has thus far been shown to accept indole as a substrate.…”

“…Naphthalene dioxygenase was soon found not to be the only enzyme capable of indole oxidation: several other oxygenases that accept indole as a substrate have been identified (5)(6)(7)(8)(9)(10). These are either enzymes similar to naphthalene dioxygenase that activate oxygen with iron centers or members of the cytochrome P450 family (6,11).…”

“…These are either enzymes similar to naphthalene dioxygenase that activate oxygen with iron centers or members of the cytochrome P450 family (6,11). Although flavin nucleotides may be involved in electron transfer from cofactors in these proteins, none is known to be a flavoprotein oxygenase.…”

CYP267A1 and CYP267B1 from Sorangium cellulosum So ce56 are Highly Versatile Drug Metabolizers

“…It was observed that indole acts as an inhibitor of CYP264A1 from S. cellulosum So ce56 and CYP109B1 from Bacillus subtilis (Girhard et al, 2010). In addition, some P450 enzymes are known to convert indole, and in this way competing with the normal substrate (Gillam et al, 2000). In case of the two members of the CYP267 family, the presence of 600 mM indole decreases the product formation in vitro by up to 40% for CYP267B1 and by up to 85% in the case of CYP267A1 (Supplemental Fig.…”

Exploiting the Versatility of Human Cytochrome P450 Enzymes: The Promise of Blue Roses From Biotechnology

“…After extensive consideration of the cause of this apparent artifact, we determined the cause of the phenomenon in collaboration with the laboratory of Prof. F. H. Arnold at the California Institute of Technology. The blue color was indigo, probably the oldest dye known to man (13,14).…”