Human enzymes produce key metabolites without gut bacteria, study reveals

A new study published in Nature Metabolism has upended long-held beliefs about the origins of indole and phenol compounds in the body. Researchers found that human enzymes can produce these metabolites independently, without relying on microbial activity. This discovery challenges the assumption that such compounds stem mainly from gut bacteria breaking down dietary tryptophan and tyrosine. The team used a multi-omic approach, combining metabolomics, genomics, and enzymology to trace the sources of indoles and phenols. Their work involved germ-free animal models and cell cultures, both free from microbial influence. This confirmed that host enzymes alone could convert tryptophan and tyrosine into these key metabolites.

Different organs showed unique enzymatic profiles and metabolic fluxes for indole and phenol synthesis. This suggests these compounds may have local, tissue-specific roles, influencing physiology through autocrine or paracrine effects. The findings also imply that host metabolism contributes significantly to circulating levels of these biomarkers, adding complexity to their use in diagnostics.

The study highlights the evolutionary advantage of such pathways, ensuring critical metabolites remain available under varying conditions. It also expands the concept of host-microbiome symbiosis, revealing a more self-sufficient metabolic capacity in humans. These insights could reshape drug development and toxicology, as host-derived metabolism may alter drug efficacy, toxicity, and interactions. Understanding these pathways may also open new treatment avenues for diseases linked to disrupted indole and phenol metabolism, such as inflammatory bowel disease and neurodegenerative disorders.