CLONING AND EXPRESSION OF COMPLEMENTARY DNAS FOR MULTIPLE MEMBERS OF THE HUMAN CYTOCHROME-P450IIC SUBFAMILY

The present study characterizes the profile of cDNAs from the human P450IIC subfamily in a library from one individual, and it describes three new members of this subfamily (IIC17, IIC18, and IIC19) isolated from two human cDNA libraries. cDNA libraries were constructed from two human livers which differed phenotypically in the hepatic content of P450 HLx (IIC8). The library from the phenotypically low HLx individual was screened by using a cDNA for rat liver P450IIC13 and an oligonucleotide probe for human IIC8. One clone, 254c, was isolated which clearly represents a new member of the human P450IIC subfamily (IIC17). This clone lacked the first 358 nucleotides at the N-terminus but was only 91% homologous in its nucleic acid sequence to IIC9 and 79% homologous to IIC8. Near-full-length clones for IIC9 were also isolated from this library, but no clones for IIC8 were found. Northern blots indicated that the mRNA for IIC8 was low or absent in this individual. A second cDNA library (from a liver phenotypically high in HLx) was then screened. Eighty-three essentially full-length (> 1.8 kb) clones belonging to the IIC subfamily were isolated from this library. These include full-length clones for two additional new members of the IIC subfamily. Clones 29c and 6b appear to be allelic variants (IIC18), differing by one nucleotide (one amino acid change) in the coding region. Clone 11a represents a full-length clone for a third new P450 (IIC19). Both IIC18 and IIC19 are most homologous to IIC17 (87% and 95%, respectively). Full-length clones were isolated for two allelic variants of IIC9 (clones 25 and 65) which differed by three nucleotides, resulting in one amino acid difference. Hybridization analysis and partial sequencing indicated that of the 83 clones in this library, 50 were 1 of the 2 allelic variants of IIC9, 29 were IIC8, 3 were the 2 allelic variants of IIC18, and 1 was IIC19. cDNAs for IIC9, IIC18, and IIC19 were expressed in COS-1 cells. Antibody to IIC9 recognized the proteins in cells transfected with cDNAs for IIC9; however, neither antibody to IIC9 or IIC8 recognized the other two cytochromes, suggesting that these cytochromes differ immunochemically from IIC8 and IIC9. COS-1 cells transfected with IIC18 showed an increase in metabolism of mephenytoin, but no metabolism of mephenytoin was detected in COS-1 cells transfected with IIC9 or IIC19.