Haptoglobin: Old protein with new functions

When released from red blood cells (RBCs), hemoglobin (Hb) is extremely toxic due in large part to the redox activity of its heme center. Nature however, has provided a multitude of protective mechanisms that can detoxify free Hb effectively under physiological conditions. Chief amongst them is haptoglobin (Hp) which chaperones Hb subunits to the macrophages for safe degradation. Recent research on the interactions between Hb and Hp under oxidative conditions revealed that Hp specifically shields key amino acids on the Hb molecule, allowing the heme to consume oxidants and short-circuits the emerging and damaging radicals. Moreover, animal studies showed that the infusion of Hb complexed with Hp prevents Hb-induced systemic hypertension and tissue injury. It may prove necessary to explore these protective clearing mechanisms to counter the toxicity associated with free Hb when used as oxygen therapeutics in hemolytic anemias and in RBC storage lesions.