UNIT 4.29 Measurement of Phenylalanine Monooxygenase (PAH) Activities

  1. Glyn B. Steventon1,
  2. Stephen C. Mitchell2

Published Online: 1 AUG 2009

DOI: 10.1002/0471140856.tx0429s41

Current Protocols in Toxicology

Current Protocols in Toxicology

How to Cite

Steventon, G. B. and Mitchell, S. C. 2009. Measurement of Phenylalanine Monooxygenase (PAH) Activities. Current Protocols in Toxicology. 41:4.29.1–4.29.11.

Author Information

  1. 1

    King's College London, London, United Kingdom

  2. 2

    Imperial College London, London, United Kingdom

Publication History

  1. Published Online: 1 AUG 2009
  2. Published Print: AUG 2009


Mammalian phenylalanine monooxygenase (phenylalaninase, phenylalanine hydroxylase, PAH; EC is a member of the large aromatic amino acid hydrolase cohort of enzymes that include tyrosine monooxygenase and tryptophane monooxygenase. PAH is a non-heme-iron-dependent protein that normally catalyzes the C-oxidation of phenylalanine (Phe) to tyrosine (Tyr) in the presence of BH4, utilizing molecular dioxygen as an additional substrate. However, over recent years, the presumed narrow substrate specificity of PAH has been questioned and catalytic activity towards alternative xenobiotic substrates (both environmental and drugs) has been reported. Like the cytochrome P450 system, PAH is able to oxidize both aliphatic and aromatic carbon centers in addition to undertaking the S-oxidation of aliphatic thioethers (including the two mucoactive drugs S-carboxymethyl-l-cysteine and S-methyl-l-cysteine). Curr. Protoc. Toxicol. 41:4.29.1-4.29.11. © 2009 by John Wiley & Sons, Inc.


  • phenylalanine monooxygenase;
  • S-oxidation;
  • S-carboxymethyl-l-cysteine