Polymorphisms in P450CYP1B1 affect the conversion of estradiol to the potentially carcinogenic metabolite 4-hydroxyestradiol

Most drug metabolizing cytochrome P450s (P450) are predominantly expressed in the liver, In contrast, human CYP1B1 is an extrahepatic P450 which is ov erexpressed in many tumours and has been strongly implicated in the activat ion of carcinogens. Rare allelic variants of the CYP1B1 gene which encode a n inactive protein have been identified. However, four polymorphisms which most likely do not abolish functionality have been described. In this repor t, we have characterized the functional consequences of these, A CYP1B1 cDN A, identical to a cDNA published previously, served as a template to introd uce allelic changes either separately or in combination. The resulting effe cts on CYP1B1 activity were determined in membranes isolated from Escherich ia coli which coexpressed CYP1B1 together with P450 reductase. None of the allelic changes affected the CYP1B1 expression level. The allelic changes A rg(48) to Gly, Ala(119) to Ser and Asn(453) to Ser had little influence on the V-max and the K-m of the CYP1B1 mediated 2- and 4-hydroxylation of estr adiol, In contrast, the K-m of these metabolic pathways was increased at le ast three-fold by the allelic change Val(432) to Leu or by simultaneously c hanging Val(432) to Leu and Asn(453) to Ser. However, these alterations had little effect on the kinetic parameters of other CYP1B1 mediated reactions such as the epoxidation of (-)-trans-(7R,8R)-benzo[a]pyrene 7,8-dihydrodio l as determined by (r-7,t-8,t-9,c-10)-benzo[a]pyrene tetraol formation, or such as the O-dealkylatlon of ethoxyresorufin and the 1'-hydroxylation of b ufuralol. Molecular modelling suggests that amino acid residue 432 of CYP1B 1 may be involved in the interaction between CYP1B1 and P450 reductase. Sin ce 4-hydroxyestradiol has been implicated in hormonal carcinogenesis and CY P1B1 is expressed in target tissues, the data presented demonstrate that po lymorphisms in CYP1B1 have the potential to affect disease susceptibility. Pharmacogenetics 10:343-353 (C) 2000 Lippincott Williams & Wilkins.