Nature Chem. Biol. doi: 10.1038/nchembio.92 (2008)

Mammals can reduce nitrate ions (NO3−) to nitrite ions (NO2−), a trick that only bacteria were thought able to perform.

Jon Lundberg of the Karolinska Institute in Stockholm and his colleagues had set out to study the importance of natural gut bacteria in nitrate metabolism using mice bred to harbour no microorganisms. But when these germ-free mice were fed sodium nitrate, and nitrite ions then showed up in their blood, the team began hunting for enzymes to explain the result.

They recorded nitrate-reducing activity from xanthine oxidoreductase in the liver tissue of both rodents and humans, and found that NO2− can be further reduced to NO. The pathway ramped up during an experiment in which the researchers clamped the abdominal aorta of rats — perhaps unsurprisingly, given that NO dilates blood vessels.