Abstract
In the present, the consequences of nitrate pollution of the environment are very pronounced. In humans and animals, microorganisms can reduce nitrates to nitrites, which cause cancer. Purified and homogeneous xanthine oxidase (XO) of cow's milk can restore these compounds, which makes the article extremely relevant. The purpose of the article is to determine the effect of antioxidants on the activity of xanthine oxidase in fresh ovine milk. Various natural and artificial antioxidants were examined for the detection of xanthine oxidase (XO) activity in ovine milk. Among the natural antioxidants, L-cysteine was more effective in the stabilization of XO in heated milk. XO of sheep milk activated by heat treatment in the presence of cysteine and molybdenum became able to convert nitrate and nitrite to nitric oxide (NO). Therefore, L-cysteine was used for double purposes: as the protector of enzyme active center against the oxidation during heat treatment of milk and as a reagent for S-nitrosothiol formation. Hypoxanthine, as a natural substrate of XO, is an effective electron donor for nitrate reductase (NR) and nitrite reductase (NiR) activities. Heat treatment of the milk in the presence of exogenous lecithin increased the activity of NR and NiR of XO and CysNO formation. Thus, during the heat treatment: a) excess of exogenous phospholipids disintegrates the structure of milk fat globule membrane (MFGM) and b) enzyme molecules denatured partially and their active center became available for exogenous cysteine, molybdenum, hypoxanthine, and nitrate or nitrite.