Abstract:
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.
