Role of nitrates in the adaptation of fish to hypoxic conditions

The paper reviews scientific and practical aspects of research on the metabolism of nitrates into nitrate (nitrite) reductase in the nitric oxide cycle in fish under hypoxic conditions. Literature data are given about enzymes involved in nitrate reductase reactions and how oxygen shortage affects their activity. The environmental factors that may participate in the processes of nitrate reduction to nitrites as well as the ones which may influence the kinetics of nitrate in fishes are considered. Examined results indicate that certain components of the conservative L-arginine-dependent nitric oxide cycle pass in a number of vertebrate animals. Then, the contour cycle of nitric oxide in the fish shows a wide range of diversity which is represented by phylogenetically ancient biochemical mechanisms of nitrite (nitrate) reductase. First of all, the presence or absence of hemoglobin is distinguished. Secondly, a wider range of expression of different myoglobin isoforms appears. Furthermore, the kinetics of nitrates in fish is distinctly dependent on abiotic environmental factors. The analysis of the literature confirms the thesis that nitrates are an important substrate for the nitrate (nitrite) reductase loop cycle of nitric oxide in fishes. Secondly, their role in the compensation of arginineindependent NO synthesis increases with decreasing oxygen levels in the environment. Thirdly, the provided research results are a basis for indicating xanthine oxidase and possibly the microflora of the digestive system as the basic units of the nitrate reductase system in the body of fish. The practical aspect of the question, in our opinion, is the most meaningful presence of numerous studies that emphasize the need to find physiological reactions that precede the formation of pathological changes induced by the influence of combined effects of nitrates and hypoxia on the organism of fish.