The monooxygenases of birds, reptiles and amphibians

1. Microsomal monooxygenase systems which contain cytochrome P-450 forms as their active centres are found in birds, reptiles and amphibians. Liver provides a rich source of monooxygenases but they are also present in other tissues. 2. In the hepatic microsomes of these species, levels of cytochrome P-450 and, in most cases monooxygenase activities, are lower than are found in hepatic microsomes of mammals. 3. Amongst birds, the lowest hepatic microsomal monooxygenase activities have been reported for specialized predators (fish-eaters and raptors). 4. When birds are dosed with inducers of the 3-methylcholanthrene (MC) type, the pattern of induction is similar to that in mammals. In contrast, phenobarbitone-type inducers are less effective in birds than in mammals--in some cases having no action at all. Prochloraz is a stronger inducer in birds than in the rat; it is an inducer of mixed type in birds. 5. Partial purification of avian cytochromes P-450 indicates substantial differences in properties from those of mammals. Further differences between birds and mammals in regard to the immunochemical properties of isoforms are evident from comparative studies using Western blotting. 6. On the evidence of Western blotting and response to inducers, cytochromes of family II (P450 II) seem less well represented in birds than in mammals. 7. The low monooxygenase activities of certain species of birds may make them relatively susceptible to lipophilic environmental chemicals that are detoxified by this system.