Enzymatic basis of the debrisoquine/sparteine-type genetic polymorphism of drug oxidation. Characterization of bufuralol 1'-hydroxylation in liver microsomes of in vivo phenotyped carriers of the genetic deficiency

Biochem Pharmacol. 1987 Dec 1;36(23):4145-52. doi: 10.1016/0006-2952(87)90573-9.


The genetically controlled polymorphic oxidation of debrisoquine and sparteine is caused by the absence or functional deficiency of a cytochrome P-450 isozyme. In order to elucidate the mechanisms underlying the differences in cytochrome P-450 function we have studied the 1'-hydroxylation of the prototype drug bufuralol in human liver microsomes of individuals phenotyped in vivo as extensive metabolizers (EM, N = 10), poor metabolizers (PM, N = 5) and in subjects with an intermediate rate of metabolism (IM, N = 4). PM- as compared to EM-microsomes were characterized by a decreased Vmax for (+)-bufuralol 1'-hydroxylation (7.51 +/- 2.03 nmol X mg-1 X hr-1 vs 11.95 +/- 4.80 nmol X mg-1 X hr-1) but not for (-)-bufuralol 1'-hydroxylation (4.72 +/- 0.87 nmol X mg-1 X hr-1 vs 5.55 +/- 1.49 nmol X mg-1 X hr-1). The apparent Km for (+)-bufuralol 1'-hydroxylation was increased in PM microsomes (118 +/- 84.9 microM vs 17.9 +/- 6.30 microM). Inhibition of bufuralol 1'-hydroxylation by quinidine was biphasic in EM microsomes, providing further support for the involvement of at least two cytochrome P-450 isozymes. Quinidine acted as a competitive inhibitor of only the high affinity/stereoselectivity component of the reaction. Our data suggest that the debrisoquine/sparteine type of oxidation polymorphism is caused by an almost complete loss of a minor cytochrome P-450 isozyme which has a high affinity and stereoselectivity for (+)-bufuralol and a high sensitivity to inhibition by quinidine.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Aged
  • Binding, Competitive
  • Cytochrome P-450 Enzyme System / deficiency*
  • Debrisoquin / metabolism*
  • Ethanolamines / metabolism*
  • Humans
  • Hydroxylation
  • Isoenzymes / deficiency*
  • Isoquinolines / metabolism*
  • Kinetics
  • Microsomes, Liver / drug effects
  • Microsomes, Liver / enzymology*
  • Middle Aged
  • Phenotype
  • Polymorphism, Genetic*
  • Quinidine / pharmacology
  • Sparteine / metabolism*


  • Ethanolamines
  • Isoenzymes
  • Isoquinolines
  • Sparteine
  • bufuralol
  • Cytochrome P-450 Enzyme System
  • Quinidine
  • Debrisoquin