Fig. 3

Analytical approaches for quantification expression and activity of tryptophan catabolising enzymes (TCEs) in cancer therapies. The levels of Trp and its metabolites can be measured by chromatographic methods, including high-performance liquid chromatography (HPLC), gas chromatography–mass spectrometry (GC-MS) or liquid chromatography–mass spectrometry (LC-MS).²⁰⁸ Antibodies detecting Trp, as well as specific metabolites of the Trp degradation pathway, enable enzyme-linked immunosorbent assay (ELISA) measurements.^{163,209,210,211} Furthermore, matrix-assisted laser desorption/ionisation (MALDI) mass spectrometry (MS) imaging allows visualisation of Trp and Kyn in tissue,^90,212 while position-emission tomography (PET) imaging with Trp-derived tracers enables visualisation of Trp uptake in vivo.^{213,214,215,216,217,218} In addition, TCE expression can be detected on mRNA and protein levels, however care must be taken when selecting anti-IDO1 antibodies, as many lack selectivity, particularly for immunohistochemistry.²⁷ The blue area of the circle illustrates methods used for measuring Trp and its metabolites; the yellow area illustrates methods used for detcting the expression of TCE and AHR activity. IF: immunofluorescence; WB: Western blot. PET-imaging example was reproduced under a Creative Commons CC-BY license from.²¹⁹ RNA-seq example was reproduced under a Creative Commons CC-BY license from.²²⁰ Western Blot example was provided by the authors.