Characterization of chicken cytochrome P450 1A4 and 1A5: Inter-paralog comparisons of substrate preference and inhibitor selectivity

The chicken (Gallus gallus) is one of the most economically important domestic animals and also an avian model species. Chickens have two CYP1A genes (CYP1A4 and CYP1A5) which are orthologous to mammalian CYP1A1 and CYP1A2. Although the importance of chicken CYP1As in metabolism of endogenous compounds and xenobiotics is well recognized, their enzymatic properties, substrate preference and inhibitor selectivity remain poorly understood. In this study, functional enzymes of chicken CYP1A4 and CYP1A5 were successfully produced in Escherichia coli (E. coli). The substrate preference and inhibitor specificity of the two chicken CYP1As were compared. Kinetic results showed that the enzymatic parameters (Km, Vmax, Vmax/Km) for ethoxyresorufin O-deethylase (EROD) and benzyloxyresorufin O-debenzylase (BROD) differed between CYP1A4 and CYP1A5, while no significant difference was observed for methoxyresorufin O-demethylase (MROD). Lower Km of CYP1A4 for BROD suggests that CYP1A4 has a greater binding affinity to benzyloxyresorufin than either ethoxyresorufin or methoxyresorufin. The highest Vmax/Km ratio was seen in BROD activity for CYP1A4 and in MROD for CYP1A5 respectively. These results indicate that substrate preference of chicken CYP1As is more notably distinguished by BROD activity and CYP1A5 prefers shorter alkoxyresorufins resembling its mammalian ortholog CYP1A2. Differential patterns of MROD inhibition were observed between CYP1As and among the five CYP inhibitors (α-naphthoflavone, furafylline, piperonyl butoxide, erythromycin and ketoconazole). α-Naphthoflavone was determined to be a potent MROD inhibitor of both CYP1A4 and CYP1A5. In contrast, no or only a trace inhibitory effect (< 15%) was observed by erythromycin at a concentration of 500 μM. Stronger inhibition of MROD activity was found in CYP1A5 than CYP1A4 by relatively small molecules α-naphthoflavone, piperonyl butoxide and furafylline. AROD kinetics and inhibition profiles between chicken CYP1A4 and CYP1A5 demonstrate that the two paralogous members of the CYP1A subfamily have distinct enzymatic properties, reflecting differences in the active site geometry between CYP1A4 and CYP1A5. These findings suggest that CYP1A4 and CYP1A5 play partially overlapping but distinctly different physiological and toxicological roles in the chicken.