Haptoglobin: the hemoglobin detoxifier in plasma.

Hemoglobin (Hb) is one of the most studied proteins. However, oxidative toxicity associated with free Hb in circulation and its contribution to inflammation and complications of transfusion have only recently become active areas of research. New insights into the protective mechanisms of haptoglobin (Hp), a plasma protein, and a timely resolution of the crystal structure of the Hb-Hp complex made it possible to definitively link the functional and structural interplay between the two proteins. Here, we summarize current knowledge of the interactions between Hb and Hp under oxidative stress conditions, and how Hb's own damaging radicals are harnessed by complex formation. Potential therapeutic benefits of using Hp for inactivation and clearance of free Hb under a number of clinical settings are considered.