Fig. (1) Physiological Nitric Oxide (NO) formation pathways. NO production using xanthine oxidoreductase (XOR) can be synchronized with nitric oxide synthase (NOS). NO turnover pathway is dependent on co-substracts as molecular oxygen (O₂) and NADPH and cofactors such flavin adenine dinucleotide (FAD), flavin mononucleotide (FMN), tetrahydrobiopterin (BH₄), heme and binding calmodulin (CAL) for NO synthesis.