Abstract
At present time the pollution of environment with nitrates is a big problem in the world. in the environment and in human and animal body microorganisms may reduce nitrates to nitrites. 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). 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. Heat treatment of the milk in the presence of exogenous lecithin increased the activity of NR and NiR of XO and CysNO formation. In result during the heat treatment: a) excess of exogenous phospholipids disintegrate the structure of MFGM and b) enzyme molecules denatured partially and their active center became available for exogenous cysteine, molybdenum, hypoxanthine and nitrate or nitrite. Cysteine not only protects SH-groups of Mo-co against oxidation but also promotes the binding of molybdenum to the cofactor.