Biological Activities of Hemopexin

Heme Transfer to Hemopexin and Direct Antioxidant Effects 

In vivo heme is released into the plasma by erythrocyte lysis in the form of hemoglobin and by tissue trauma in the form of myoglobin, and both heme proteins are quickly oxidized to their ferric heme forms (methemoglobin and metmyoglobin) at the oxygen tension found in tissue capillary beds. 

Haptoglobin binds hemoglobin in a 1 1 molar complex (two ap dimers per haptoglobin) (42). This complex is quickly removed from the circulation via a suicidal receptor-mediated endocytosis (43,44), which consequently depletes haptoglobin. Even a short bout of exercise can deplete haptoglobin (45), whose total plasma amount is equivalent to the hemoglobin in about 4.5 cm of red cells. 

The kidney is known to be particularly vulnerable to the toxic effects of heme and hemoglobin in disseminated intravascular hemolysis, and 

Heme dissociates from methemoglobin or metmyoglobin in the circulation and can be boimd by hemopexin or albumin, a heme binding plasma protein of lower avidity than hemopexin (49). It is important that the heme be controlled, since this amphipathic, oxidatively active compound can nonspecifically associate with membrane lipids or lipoproteins and cause oxidative damage of vital biomolecules, including DNA (50, 51). 

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