Oxidation of indole by cytochrome P450 enzymes

Biochemistry. 2000 Nov 14;39(45):13817-24. doi: 10.1021/bi001229u.


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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Aryl Hydrocarbon Hydroxylases*
  • Cytochrome P-450 CYP2A6
  • Cytochrome P-450 Enzyme System / chemistry
  • Cytochrome P-450 Enzyme System / genetics
  • Cytochrome P-450 Enzyme System / metabolism*
  • Escherichia coli / enzymology
  • Escherichia coli / genetics
  • Escherichia coli / metabolism
  • Humans
  • Indigo Carmine
  • Indoles / chemistry
  • Indoles / metabolism*
  • Isatin / chemistry
  • Male
  • Microsomes, Liver / enzymology
  • Microsomes, Liver / metabolism
  • Mixed Function Oxygenases / chemistry
  • Mixed Function Oxygenases / genetics
  • NADH, NADPH Oxidoreductases / chemistry
  • NADH, NADPH Oxidoreductases / genetics
  • NADH, NADPH Oxidoreductases / metabolism
  • NADPH-Ferrihemoprotein Reductase / chemistry
  • NADPH-Ferrihemoprotein Reductase / genetics
  • NADPH-Ferrihemoprotein Reductase / metabolism
  • Oxidation-Reduction
  • Pigments, Biological / chemistry
  • Rats
  • Rats, Wistar
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / metabolism


  • Indoles
  • Pigments, Biological
  • Recombinant Proteins
  • indoxyl
  • Isatin
  • Cytochrome P-450 Enzyme System
  • Indigo Carmine
  • Mixed Function Oxygenases
  • Aryl Hydrocarbon Hydroxylases
  • Cytochrome P-450 CYP2A6
  • NADH, NADPH Oxidoreductases
  • NADPH-Ferrihemoprotein Reductase
  • indirubin