cjs-202305-0003 ArticleInAPeriodical 2023 Czech Journal of Animal Science 68 5 222-230 AubakirovaKarlygash SatkanovMereke KulataevaMaral AssylbekovaGulmira KambarbekovaAigul AlikulovZerekbai Molybdoenzymes isolated from S. glanis liver can produce nitric oxide from nitrates and nitrites Nitric oxide (NO) plays numerous essential physiological functions in terrestrial animals. In mammals, NO production from l-arginine is catalysed by the enzyme NO synthase (NOS). In recent years, data have begun to emerge on NOS expression and the physiological significance of NO in ectothermic vertebrates such as fish. However, there are relatively fewer data compared to the mammalian system. Although it is already well known that animal molybdoenzymes can convert nitrate and nitrite into NO, there is almost no information on the content and properties of molybdoenzymes in fish organs in the scientific literature. In this regard, the objectives of the present work were to detect the activity of classical molybdoenzymes xanthine oxidase (XO) and aldehyde oxidase (AO) in the liver and to study their possible activity to reduce nitrate and nitrite to nitrogen monoxide. In this work, the intrinsic activity of XO and AO was examined by using their substrates. At the same time, their nitrate (NR) - and nitrite reductase (NiR) activity were determined. It was determined that XO and AO in the fish liver are mainly represented by the molybdenum-free forms. The presence of an additional source of molybdenum can activate both the intrinsic and the NR and NiR activities. The NiR activity of XO and AO was higher than their NR activity. The data shows that treatment at a concentration of sodium molybdate and glutathione of 1.0 mM increases all activities of XO and AO. The optimal conditions for maximum activation of exogenous molybdate for XO and AO were reached by heating at 70 °C for 5 minutes. The activity of XO increased almost 4.7 times, and the activity of AO 7.7 times compared with its intrinsic activity without heat treatment. NO is formed from nitrite by the enzymes XO and AO much more than from nitrate.