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Biotransformation of Xenobiotics

Basic Science

  1. John A. Timbrell PhD, DSc, MRCPath, FIBiol, FRSC Emeritus Professor of Biochemical Toxicology1,
  2. Timothy C. Marrs OBE, MD, DSc, FRCP, FRCPath, FATS, FBTS2,3

Published Online: 15 DEC 2009

DOI: 10.1002/9780470744307.gat004

General, Applied and Systems Toxicology

General, Applied and Systems Toxicology

How to Cite

Timbrell, J. A. and Marrs, T. C. 2009. Biotransformation of Xenobiotics. General, Applied and Systems Toxicology. .

Author Information

  1. 1

    King's College London, Pharmacology and Therapeutics Research Group, Pharmaceutical Sciences Research Division, London, UK

  2. 2

    Edentox Associates, Edenbridge, Kent, UK

  3. 3

    City Hospital, National Poisons Information Service (Birmingham Centre), Birmingham, UK

Publication History

  1. Published Online: 15 DEC 2009


Xenobiotics, which are absorbed into biological systems by passive diffusion across membranes are usually lipid soluble and not ideally suited for excretion. Indeed very lipophilic substances may remain in the mammalian body for many years. After a xenobiotic has been absorbed, it may undergo biotransformation to products which are rapidly excreted and therefore elimination of the compound from the animal is facilitated. However, biotransformation may also change the biological activity of the substance. Hence, the metabolic fate of the compound can have an important bearing on its toxic potential, the disposition of the compound in the body and the excretion of the compound. The metabolic reactions involved are usually divided into phase 1 and phase 2 reactions, the latter being conjugation reactions. The products of metabolism are usually more water soluble than the original compound. Although usually detoxifying, these reactions, especially phase 1 ones, sometimes increase toxicity.


  • biotransformation;
  • metabolism;
  • Phase 1;
  • Phase 2;
  • conjugation;
  • cytochrome;
  • hydroxylation;
  • oxidation;
  • dealkylation