Haptoglobin is the Protein That Binds Free Hemoglobin in Plasma
Haptoglobin is the plasma protein with the highest binding affinity for hemoglobin and is primarily responsible for binding free hemoglobin in the plasma. This critical function prevents hemoglobin-mediated oxidative damage and renal injury following hemolysis.
Haptoglobin's Role in Hemoglobin Binding
Haptoglobin functions as the body's primary defense mechanism against free hemoglobin through several mechanisms:
- It binds free hemoglobin with extremely high affinity, forming stable haptoglobin-hemoglobin complexes 1, 2
- This binding prevents glomerular filtration of hemoglobin, protecting the kidneys from iron-induced damage 1
- The haptoglobin-hemoglobin complex is recognized by the CD163 receptor on macrophages, facilitating clearance from circulation 3
Evidence Supporting Haptoglobin as the Answer
Research clearly demonstrates that haptoglobin is specifically evolved for hemoglobin binding:
- Haptoglobin is described as "the plasma protein with the highest binding affinity for hemoglobin" 1
- It plays a "critical role in tissue protection and prevention of oxidative damage" by binding free hemoglobin 2
- Haptoglobin deficiency leads to "increased deposition of hemoglobin in proximal tubules of the kidney" rather than proper clearance through the liver and spleen 1
Distinguishing from Other Plasma Proteins
While other proteins mentioned in the question have important plasma functions, they do not primarily bind free hemoglobin:
- Hemopexin: Binds free heme (not intact hemoglobin) that is released after hemoglobin oxidation
- Prealbumin (Transthyretin): Primarily transports thyroid hormones and vitamin A
- Transferrin: Binds and transports iron in plasma, but not hemoglobin
Clinical Significance
Understanding haptoglobin's role has important clinical implications:
- Haptoglobin levels decrease during hemolysis as it binds and clears free hemoglobin
- Haptoglobin is an acute-phase protein whose concentration changes in various pathological conditions 2
- Haptoglobin phenotypes (Hp1-1, Hp2-1, Hp2-2) may influence individual predisposition to various diseases 2
- Haptoglobin-related protein (Hpr) also binds hemoglobin with high affinity but does not promote binding to CD163 receptors 4
The correct answer is A. Haptoglobin.