Hemolysis and free hemoglobin revisited: exploring hemoglobin and hemin scavengers as a novel class of therapeutic proteins

Abstract

Hemolysis occurs in many hematologic and nonhematologic diseases. Extracellular hemoglobin (Hb) has been found to trigger specific pathophysiologies that are associated with adverse clinical outcomes in patients with hemolysis, such as acute and chronic vascular disease, inflammation, thrombosis, and renal impairment. Among the molecular characteristics of extracellular Hb, translocation of the molecule into the extravascular space, oxidative and nitric oxide reactions, hemin release, and molecular signaling effects of hemin appear to be the most critical. Limited clinical experience with a plasma-derived haptoglobin (Hp) product in Japan and more recent preclinical animal studies suggest that the natural Hb and the hemin-scavenger proteins Hp and hemopexin have a strong potential to neutralize the adverse physiologic effects of Hb and hemin. This includes conditions that are as diverse as RBC transfusion, sickle cell disease, sepsis, and extracorporeal circulation. This perspective reviews the principal mechanisms of Hb and hemin toxicity in different disease states, updates how the natural scavengers efficiently control these toxic moieties, and explores critical issues in the development of human plasma-derived Hp and hemopexin as therapeutics for patients with excessive intravascular hemolysis.

Figure 1

Schematic summary of the Hb clearance compartments and the main acute and chronic pathologies that can be associated with intravascular hemolysis. The availability of the Hb and hemin scavenger proteins Hp and Hpx shifts the physiologic balance from tissue damage toward protection.

Figure 2

Schematic summary of the principal mechanisms of Hb toxicity and protection by the plasma scavenger proteins Hp and Hpx.

Figure 3

Hp sequestration of Hb. Guinea pigs were infused with stroma-free Hb (peak plasma Hb concentration, 150μM heme) and, after 10 minutes, a treatment group of animals was infused with human plasma–derived Hp to match an equimolar Hb:Hp concentration. (A) Mean arterial blood pressure response before and after Hp treatment. (B) Unbound plasma Hb (red) before and after Hp administration. The left shift in the chromatogram indicates the large molecular size Hb:Hp complex with approximately 90% Hp bound and 10% unbound Hb. (C) Hemoglobinuria after Hb infusion (150μM heme) without (−Hp, left) and with Hp (+Hp, right). (D) Iron deposition (brown staining) in normal kidney renal cortex (left), Hb infusion (middle), and Hb infusion plus Hp (right). (E) HO-1 expression in kidneys after 24 hours after Hb exposure with and without Hp infusion (left). Densitometry is shown to the right of the HO-1 Western blot. All data are presented as means ± SEM.