A novel CYP2A6 allele, CYP2A6*23, impairs enzyme function in vitro and in vivo and decreases smoking in a population of Black-African descent

Pharmacogenet Genomics. 2008 Jan;18(1):67-75. doi: 10.1097/FPC.0b013e3282f3606e.


Objectives: CYP2A6 is the main enzyme involved in nicotine metabolism in humans. We have identified a novel allele, CYP2A6*23 (2161C>T, R203C), in individuals of Black-African descent and investigated its impact on enzyme activity and association with smoking status.

Methods: Wild-type and variant enzymes containing amino acid changes R203C (CYP2A6*23), R203S (CYP2A6*16) and V365M (CYP2A6*17) were expressed in Escherichia coli. The effect of CYP2A6*23 in vivo was examined in individuals of Black-African descent given 4 mg oral nicotine.

Results: CYP2A6*23 occurred at an allele frequency of 2.0% in individuals of Black-African descent (N=560 alleles, 95% confidence interval, 0.8-3.1%) and was not detected in Caucasians (N=334 alleles), Chinese (N=288 alleles) or Japanese (N=104 alleles). In vitro, CYP2A6.23 had greatly reduced activity toward nicotine C-oxidation similar to CYP2A6.17, as well as reduced coumarin 7-hydroxylation. Conversely, CYP2A6.16 did not differ in activity compared with the wild-type enzyme. The trans-3'-hydroxycotinine to cotinine ratio, a phenotypic measure of CYP2A6 activity in vivo, was lower in CYP2A6*1/*23 and CYP2A6*23/*23 individuals (mean adjusted ratio of 0.60, n=5) compared with CYP2A6*1/*1 individuals (mean adjusted ratio of 1.21, n=150) (P<0.04). CYP2A6*23 trended toward a higher allele frequency in nonsmokers (3.1%, N=9/286 alleles) compared with smokers (0.7%, N=2/274 alleles) (P=0.06).

Conclusion: These results suggest the novel CYP2A6*23 allele impairs enzyme function in vitro and in vivo and trends toward an association with lower risk of smoking.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • African Continental Ancestry Group*
  • Alleles*
  • Aryl Hydrocarbon Hydroxylases / genetics*
  • Aryl Hydrocarbon Hydroxylases / metabolism
  • Base Sequence
  • Cytochrome P-450 CYP2A6
  • DNA Primers
  • Female
  • Gene Frequency
  • Humans
  • Male
  • Mixed Function Oxygenases / genetics*
  • Mixed Function Oxygenases / metabolism
  • Molecular Sequence Data
  • Smoking / genetics*


  • DNA Primers
  • Mixed Function Oxygenases
  • Aryl Hydrocarbon Hydroxylases
  • CYP2A6 protein, human
  • Cytochrome P-450 CYP2A6