The relationship between uric acid and its oxidative product allantoin: a potential indicator for the evaluation of oxidative stress in birds
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Uric acid is the main nitrogenous waste product in birds but it is also known to be a potent antioxidant. Hominoid primates and birds lack the enzyme urate oxidase, which oxidizes uric acid to allantoin. Consequently, the presence of allantoin in their plasma results from non-enzymatic oxidation. In humans, the allantoin to uric acid ratio in plasma increases during oxidative stress, thus this ratio has been suggested to be an in vivo marker for oxidative stress in humans. We measured the concentrations of uric acid and allantoin in the plasma and ureteral urine of white-crowned sparrows (Zonotrichia leucophrys gambelii) at rest, immediately after 30 min of exercise in a hop/hover wheel, and after 1 h of recovery. The plasma allantoin concentration and the allantoin to uric acid ratio did not increase during exercise but we found a positive relationship between the concentrations of uric acid and allantoin in the plasma and in the ureteral urine in the three activity phases. In the plasma, the slope of the regression describing the above positive relationships was significantly higher immediately after activity. We suggest that the slope indicates the rate of uric acid oxidation and that during activity this rate increases as a result of higher production of free radicals. The present study demonstrates that allantoin is present in the plasma and in the ureteral urine of white-crowned sparrows and therefore might be useful as an indicator of oxidative stress in birds.
KeywordsOxidative stress Antioxidation Free radicals Allantoin Uric acid White-crowned sparrow
We wish to thank Prof. Carlos Martínez del-Rio for his help and involvement throughout the study including his advices in the manuscript preparation and his help in the statistical analysis. Also thanks to Edwin Price, Quentin Hays, David Cerasale for their assistance in catching and maintaining the birds and in blood sampling, and for their kind hospitality, and Bradley H. Bakken for his assistance in analyzing the samples. We also thank three anonymous reviewers whose comments greatly improved this manuscript. The study was partially financed by grants to C. Martínez del-Rio (NSF IBN-0110416) and to C.G. Guglielmo (NSF IBN-0224954), by the Technion’s J. and A. Taub Biological Research Fund and J. S. Frankford Research Fund (to Zeev Arad), and by the Technion-University of Haifa Interuniversity Research Fund (to Zeev Arad and Ido Izhaki).
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