Clearance and control mechanisms of hemoglobin from cradle to grave.

Hemoglobin is a highly reactive molecule, and besides its oxygen-carrying capacity, it has multiple enzymatic and ligand-binding activities that have only recently been explored as fundamental pathophysiologic mechanisms. Nitric oxide neutralization, generation of potentially toxic radical species, and heme-mediated inflammation are among the most extensively studied mechanisms of Hb-mediated pathology. Extracellular Hb has an established role in sickle cell disease and other hemolytic disorders. However, extracellular Hb seems also to have relevant disease-modifying activities in many other important pathologic conditions, such as malaria and atherosclerosis. In this Forum, we summarize the current knowledge of mechanisms of Hb toxicity. Special emphasis is given to the highly efficient endogenous scavenger and detoxification pathways, such as alpha-hemoglobin stabilizing protein (AHSP), haptoglobin, hemopexin, CD163, and heme oxygenase. Systemic and local activity of these pathways finally determines the impact of extracellular Hb on physiology and tissue homeostasis.