However, beyond this “prokaryotic pathway” of nitrite generation in blood and tissue an “eukaryotic pathway” has also been recently described by Lundberg’s group: also mammalian cells are capable of nitrate reduction to nitrite via the involvement of a nitrate reducing enzyme, XOR [12]. Nitrite accumulation in blood and tissues represent a biological pool for NO generation since several different mammalian enzymes and metalloproteins possess nitrite reductase activity such as xanthine oxidoreductase (XOR), aldehyde oxidase (AO), heme proteins and mitochondrial respiratory chain enzymes [11], [12] and [13].