Molecular cloning of two TCDD-Induced chicken cyp1a related enzymes distinct from mammalian CYP1A1 and 1A2
Cytochrome P-450 Enzyme System
cDNAs for two catalytically distinct cytochrome P450 (CYP) enzymes previously purified from livers of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated chick embryos (designated TCDDAA and TCDDAHH were isolated and sequenced. The full size TCDDAA cDNA contains 1838 base pairs (bp) with 5' and 3' untranslated regions (UTR) of 147 and 107 bp, respectively. It codes for a 528 amino acid protein. The 2003 bp obtained for TCDDAHH contained a 3'UTR of 425 bp and 1578 bp of the coding region. Transcripts of 1.9 kb for TCDDAA and 2.3 kb for TCDDAHH were detected on Northern blots of mRNA from TCDD-treated but not from control livers. In their coding regions TCDDAA and TCDDAHH have 80.3% identity for nucleic acids and 78.5% for amino acids. They have 53.4 to 62.4% identities to 6 mammalian CYP1A1 & 1A2 enzymes, 32.7 to 34.7% to 2 1B1 enzymes, and 53.6 to 57.6% to 4 fish CYP1A enzymes. Both chick enzymes were more homologous to CYP1A1 than 1A2, TCDDAA more so than TCDDAHH. Immunochemically and catalytically, however, TCDDAA is more like CYP1A2 and TCDDAHH more like 1A1. Phylogenetic analysis placed the chick CYP1A enzymes on a separate branch of the CYP1A family tree, distinct from the fish and mammalian branches. The findings demonstrate that there are two TCDD-induced avian CYP genes; both are members of the CYP1A family but neither is the ortholog of mammalian CYP1A1 or 1A2. CYP1A precursor enzymes appear to have evolved into two functionally analogous pairs of enzymes at separate times in evolution, a possible example of convergent evolution in the CYP1 family. NIH grants ES03606.HL34318.