Antioxidant enzymes in the developing lungs of egg-laying and metamorphosing vertebrates

The activities of the pulmonary antioxidant enzymes (AOE), superoxide dismutase (SOD), glutathione peroxidase (GPx) and catalase, increase in the final 10-20 % of gestation in the mammalian lung, to protect the lung from attack by increasing levels of reactive oxygen species at birth. Whether the increase occurs as a normal 'preparation for birth', i.e. by a genetically determined mechanism, or in response to increased levels of oxygen, i.e. in response to the environment, is not completely understood. We examined the activities of catalase, SOD and GPx in the developing lungs of two oviparous vertebrate species, the chicken (Gallus gallus) and an agamid lizard (Pogona vitticeps), and in a metamorphosing vertebrate, the anuran Limnodynastes terraereginae. During in ovo development embryos come into contact with higher levels of environmental oxygen, and at a much earlier stage of development, compared with the intrauterine development of mammals. Furthermore, in metamorphosing frogs, the lungs are inflated at an early stage to aid in buoyancy, although the gas-exchange function only develops much later upon final metamorphosis. Here, we hypothesise that the activity of the AOE will be elevated relatively much earlier during development in both oviparous and metamorphosing vertebrates. We also examined the effect of mild hypoxia (17 % oxygen) on the development of the pulmonary AOE in the chicken, to test the hypothesis that these enzymes are responsive to environmental oxygen. In the normoxic lung of both Gallus gallus and Pogona vitticeps, catalase and GPx activities were significantly increased in late incubation, whereas SOD activity decreased in late incubation. Catalase and SOD activities were virtually identical in hypoxic and normoxic embryos of the chicken, but GPx activity was significantly affected by hypoxia. In the developing frog, the activities of all enzymes were high at stage 30, demonstrating that the system is active before the lung displays any significant gas-exchange function. SOD and GPx activity did not increase further with development. Catalase activity increased after stage 40, presumably correlating with an increase in air-breathing. In summary, catalase expression in the two oviparous vertebrates appears to be completely under genetic control as the activity of this enzyme does not change in response to changes in oxygen tension. However, in tadpoles, catalase may be responsive to environmental oxygen. SOD also appears to follow a largely genetically determined program in all species. Under normoxic conditions, GPx appears to follow a genetically determined developmental pattern, but this enzyme demonstrated the largest capacity to respond to environmental oxygen fluctuations. In conclusion, it appears that the AOE are differentially regulated. Furthermore, the AOE in the different species appear to have evolved different levels of dependency on environmental variables. Finally, the late developmental increase in AOE activity seen in mammals is not as pronounced in oviparous and metamorphosing vertebrates.