A single amino acid substitution (Leu160His) in cytochrome P450 CYP2A6 causes switching from 7-hydroxylation to 3-hydroxylation of coumarin

Food Chem Toxicol. 1997 Sep;35(9):903-7. doi: 10.1016/s0278-6915(97)00066-5.


Human populations are thought to metabolize coumarin almost exclusively by 7-hydroxylation. We have identified an individual who is homozygous for a single amino acid substitution (Leu160His) in the cytochrome P450 CYP2A6 arising from the variant CYP2A6*2 allele. On administration of coumarin (2 mg orally) no detectable 7-hydroxycoumarin was excreted in the 0-8-hr urine, rather, approximately 50% of the dose was eliminated as 2-hydroxyphenylacetic acid, the end-product of coumarin 3-hydroxylation. His immediate family members, who were heterozygous for the CYP2A6*2 allele, excreted little 2-hydroxyphenylacetic acid and mainly 7-hydroxycoumarin, when similarly tested. These findings raise a question regarding human risk evaluations for environmental coumarin exposures, since 7-hydroxylation is regarded as a detoxication pathway, but 3-hydroxylation as the process required to lead to macromolecular covalent binding of coumarin. Persons homozygous for the CYP2A6*2 allele may constitute 1-25% of various populations.

MeSH terms

  • Adolescent
  • Adult
  • Aryl Hydrocarbon Hydroxylases*
  • Child
  • Coumarins / metabolism*
  • Cytochrome P-450 CYP2A6
  • Cytochrome P-450 Enzyme System / genetics*
  • Cytochrome P-450 Enzyme System / metabolism
  • Female
  • Humans
  • Hydroxylation
  • Male
  • Middle Aged
  • Mixed Function Oxygenases / genetics*
  • Mixed Function Oxygenases / metabolism
  • Mutation
  • Polymorphism, Genetic
  • Structure-Activity Relationship


  • Coumarins
  • Cytochrome P-450 Enzyme System
  • coumarin
  • Mixed Function Oxygenases
  • Aryl Hydrocarbon Hydroxylases
  • CYP2A6 protein, human
  • Cytochrome P-450 CYP2A6