TDO2 and neoplasm: Nonetheless, many other compounds can act as AHR agonists and promote carcinogenesis.33 One specific activity of the microbiome that was found to be over-represented at tumour and tumour-adjacent buccal sites in OSCC patients was tryptophanase, which catalyses the degradation of l-tryptophan to indole, pyruvate and ammonium, all of which may be metabolised into agonists for AHR and potentially contribute to carcinogenesis.