Mechanism and Physiological Relevance of Xanthine Oxidase as Nitrate Reductase in Mammalian Organism: An Overview

Abstract

In living organisms nitric oxide radical is produced by the enzymes NO synthases, with consequent oxidation to nitrate by several oxidation/reduction pathways. The end step of NO oxidation is the formation of nitrite and nitrate, which after their formation enter a reduction phase of NO cycle, thereby playing a physiologically important role of NO donors. However, while in plants and microorganisms the key pathway of nitrate/nitrite reduction is the functioning of the enzymes nitrate reductases (NRs), mammalian organism lacks NRs enzymes. Instead, in mammals, the reduction of nitrates and nitrites occurs by several other, both enzymatic and non-enzymatic, pathways. Based on the present knowledge, one of the key enzymes involved in the nitrate/nitrite reduction process is xanthine oxidoreductase (XOR). XOR is capable of reducing organic and inorganic nitrates and nitrite, and the role of XOR in NO3 - → NO2 - → ·NO pathway is most studied as compared to other enzymes with nitrate reductase activity. The functions of XOR, the mechanisms of nitrate/nitrite reduction by this enzyme and their physiological relevance will be outlined in this mini review.