Topics in Xenobiochemistry: do metabolic pathways exist for xenobiotics? The micro-metabolism hypothesis.

1. The relevance of the concept of 'the metabolic pathway' for the understanding of xenobiotic metabolism is discussed in the light of advances in modern analytical methods that have enabled the detection and identification of minor metabolites present at ever lower concentrations. 2. A model is suggested where the overall metabolic fate of a xenobiotic is the sum of all the possible metabolic reactions permitted by the solution chemistry of the compound modulated by factors such as the metabolizing enzyme complement of the organism, the affinity of those enzymes for the xenobiotic substrates and the probabilities of all of these processes. 3. In this probabilistic, rather than deterministic, system, the resulting proportions of particular metabolites will, therefore, depend on the sums of the probabilities of particular biotransformation reactions occurring and the stability (chemical or metabolic) of the resulting metabolite. 4. In this model, all the potential metabolic possibilities that could result for any individual xenobiotic will occur to some extent. However, in actuality, many of the resulting metabolites will be produced or excreted in such small quantities as to defy ready detection with current methods.