Construction of expression system of rabbit aldehyde oxidase cDNA for the clarification of species differences.

A remarkably large species difference in cinchonidine oxidation activity catalyzed by aldehyde oxidase (AO) has been known, in particular between rabbit and monkey. As the first step in clarifying the phenomenon from the view point of structures of the active site, we attempted to construct an expression system of rabbit AO cDNA. The nucleotide sequences of cloned full-length rabbit AO cDNA were determined and confirmed to agree completely with those of genome DNA. The expression system in Escherichia coli was constructed in reference to the previously established method for monkey AO. Both expressed rabbit and monkey AO proteins correctly reproduced the remarkable species differences observed in their liver cytosols towards cinchonidine and methotrexate. Namely, the expressed rabbit AO protein showed extremely high activities than did that of monkey AO. A difference in the structure of the active site might be responsible for the substrate-dependent species difference towards the relatively bulky molecules of cinchonidine and methotrexate. The use of molecular biology techniques will be very useful to verify the hypothesis.