Phylogenetic distribution of the hepatic enzyme system for reducing naloxone to 6α- and 6β-naloxol in vertebrates

1. 1. Livers from a variety of vertebrate species were examined for reduction of naloxone to 6α- and 6β-naloxol. 2. 2. Bird and reptile livers possessed relatively high rates of enzyme activity and formed predominantly 6α-naloxol. 3. 3. Livers from fish and amphibia showed generally low rates of naloxone reduction and more equal amounts of 6α- and 6β-naloxol. 4. 4. Naloxone reduction in mammal livers was intermediate in activity and resulted primarily in 6β-naloxol even though guinea pigs were a particularly interesting exception. 5. 5. It was not possible to relate product stereoselectivity to phylogenetic development.