Comparative biochemistry of drug metabolism by azo and nitro reductase.

The activity of enzyme systems catalyzing the metabolism of drugs and other chemicals foreign to the body varies with the species, sex, genetic background, and the physiologic state of the animal. A variety of drugs and chemicals which cause increases in the activity of microsomal drug-metabolizing enzymes have been reported.1' 2 There is a decrease or lack of oxidative drug-metabolizing enzyme activity associated with the following conditions: (1) newborn state, (2) starvation, (3) alloxan-induced diabetes, (4) hepatic tumors or regeneration.3-7 In some of these conditions azo and nitro reductase activity were not studied, and in others there were differences between the oxidative enzymes that metabolize drugs and azo and nitro reductase. Thus, in the starved animal there may be a stimulation of enzymes reducing azo and nitro groups in contrast to a depression of oxidative enzymes that metabolize drugs.4 Moreover, the reductive enzymes are present, albeit to a small degree of the normal activity, in various liver tumors and in the fetus and newborn animals, whereas the oxidative enzymes are absent.6' 8 These results suggest that the enzymes which reduce azo and nitro groups may develop before the oxidative drug-metabolizing enzymes and may be more primitive. In a comparative study, it was demonstrated that fish and amphibians lacked oxidative microsomal drug enzymes. It was pointed out that these enzymes were not required by these water-dwelling species since the potentially toxic foreign lipidsoluble substances ingested could diffuse through the membranes of the gills or through the skin into the surrounding aqueous medium. However, the higher vertebrates having gone from water to land life in the process of evolution could no longer rid themselves of toxic lipid-soluble foreign compounds in this manner. As a prerequisite for life, land animals had to develop nonspecific enzyme systems to convert these compounds to less lipid-soluble derivatives which are more rapidly excreted by the kidney.9 The present problem was undertaken to follow the evolution of the metabolism of foreign compounds by azo and nitro reductase, since a comparative study of reductive enzymes that metabolize drugs has not been reported, and because these enzymes may be of a more primitive nature. Materials and Methods.-Lemon sharks (Negaprion brevirostris), sting rays (Dasyatis americana), barracuda (Sphyraena barracuda), and yellow-tail snappers (Ocyurus chrysurus) were caught near Bimini, Bahamas. Frogs (Rana pipiens and R. catesbeiana), toads (Bufo marinus), turtles (Pseudemys), horned toads (Phrynosoma), and common domestic pigeons were obtained from the Lemberger Company, Oshkosh, Wisconsin. Mice, rats, and guinea pigs were obtained from the Animal Production Section, National Institutes of Health. Preparation of tissue samples: Livers were homogenized in three parts of cold