of Work: Xenobiotics/steroid metabolizing enzymes, cytochromes P450 and sulfotransferases exhibit remarkably diverse substrate and product specificities. We have employed protein crystallography in order to provide structural insight into understanding these broad specificities. In addition to the estrogen sulfotransferase (EST) structure complexed with PAP and estradiol, we have now solved the EST-PAP vanadate structure and have provided the transition state of an in-line sulfuryl transfer reaction. We have also determined the structure of heparan sulfate sulfotransferase (NST) complexed with PAP. A comparison motifs are very well conserved. As expected, on the other hand, the substrate binding sites display different structures between these two enzymes. Characteristically, the hepatic P450 genes are induced by xenobiotics as a cellular defense mechanism against toxicity and carcinogenicity of xenobiotics. We have defined 51-bp phenobarbital-responsive enhancer sequence (designated PBREM) that is conserved in human, rat and mouse CYP2B genes. PBREM is found to be regulated by the nuclear orphan receptor CAR-RXR heterodimer via its direct binding to NR1 and NR2 sites within PBREM sequence. Depended on phenobarbital induction, cytosolic CAR is rapidly translocated into nuclei, forms a heterodimer with RXR and activates PBREM activity. In a stable HepG2 cell transfected with CAR expression vector, endogenous human CYP2B6 gene is induced by phenobarbital. As a result, we have identified nuclear orphan receptor CAR as a major factor that mediates phenobarbital inducible signal to CYP2B genes.
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