Structural identification of diindole agonists of the aryl hydrocarbon receptor derived from degradation of indole-3-pyruvic acid

Aerobic incubation of the tryptophan transamination/oxidation product indole-3-pyruvic acid (I3P) at pH 7.4 and 37°C yielded products with activity as Ah receptor (AHR) agonists. The extracts were fractionated using HPLC and screened for AHR agonist activity. Two compounds were identified as agonists: 1,3-di(1H-indol-3-yl)propan-2-one (1) and 1-(1H-indol-3-yl)-3-(3H- indol-3-ylidene) propan-2-one (2), with the potency of 2 being 100-fold > 1 [Nguyen et al. (2009) Chem. Res. Toxicol., DOI: 10.1021/tx900043s.]. Both 1 and 2 showed UV spectra indicative of indole. The molecular formulas were established by high-resolution mass spectrometry (HRMS), and the structures were determined by a combination of NMR methods, including ¹H, natural abundance ¹³C, and two-dimensional methods. An intermediate in the oxidation of I3P to 1 is 3-hydroxy-2,4-di(1H-indol-3-yl)butanal (HRMS established the presence of a compound with the formula C₂₀H ₁₉N₂O₂). Compound 1 was converted to 2 in air or (faster) with mild oxidants, and 2 could be further oxidized to 1,3-di(3H-indol-3-ylidene)propan-2-one. Determination of the structures allowed estimation of the molar Ah receptor agonist activity of these natural products, similar in potency to known classical AHR inducers.