Hydrogen peroxide-dependent conversion of nitrite to nitrate as a crucial feature of bovine milk catalase.

The enzyme catalase is well-known to catalyze the disintegration of hydrogen peroxide to water and oxygen; however, this study shows that its main function in bovine milk is oxidation of nitrite to nitrate. This process depends on hydrogen peroxide, of which the main source appears to be hydrogen peroxide formation that is coupled to the conversion of purines--xanthine in the present study--to uric acid by milk xanthine oxidase. However, additional secondary sources of hydrogen peroxide appear to be important during the relatively long storage of milk in the gland cistern. This paper demonstrates that the oxidation of nitrite to nitrate is necessary to prevent accumulation of free radicals and oxidative products during storage of milk in the gland and during the unavoidable delay between milking and pasteurization in dairy plants. Recommendations for minimizing the deterioration in milk quality during commercial storage are presented.