Nitrate acts as a poison by interfering with oxidative enzymes. oxidizing hemoglobin to methemoglobin, which cannot carry oxygen. reacting with enzymes that break down toxic wastes in the liver. blocking cell protein synthesis.

Methemoglobin (metHb) is an oxidized derivative of hemoglobin in which heme iron is in the ferric (Fe3+) or oxidized state rather than the ferrous (Fe2+) or reduced state. Small amounts of methemoglobin normally are formed daily, associated with the release of oxygen from hemoglobin (auto-oxidation).

Methemoglobin cannot bind oxygen, which means it cannot carry oxygen to tissues. It is bluish chocolate-brown in color. In human blood a trace amount of methemoglobin is normally produced spontaneously, but when present in excess the blood becomes abnormally dark bluish brown. The NADH-dependent enzyme methemoglobin reductase (a type of diaphorase) is responsible for converting methemoglobin back to hemoglobin.

Normally one to two percent of a person's hemoglobin is methemoglobin; a higher percentage than this can be genetic or caused by exposure to various chemicals and depending on the level can cause health problems known as methemoglobinemia. A higher level of methemoglobin will tend to cause a pulse oximeter to read closer to 85% regardless of the true level of oxygen saturation. An abnormal increase of methemoglobin will increase the oxygen binding affinity of normal hemoglobin, resulting in a decreased unloading of oxygen to the tissues.