Normally, intravascular hemolysis accounts for approximately 10% of total red blood cell destruction. Whereas in vitro hemolysis produces incorrect laboratory results, the in vivo release of free hemoglobin from red blood cells triggers a chain of damaging physiologic effects. First, free hemoglobin binds nitric oxide and clears this important vasodilator. Second, free hemoglobin will undergo oxidation, which results in the formation of methemoglobin, free radical globin-ferryl heme, and superoxide ions. The latter can undergo dismutation to form hydrogen peroxide. Further interaction of hydrogen peroxide with either deoxy- or oxyhemoglobin can result in the formation of ferryl hemoglobin and even more superoxide.
One of the major protective mechanisms in plasma is the protein haptoglobin. Free hemoglobin dimers bind to haptoglobin with very high affinity, and these complexes are rapidly removed by a macrophage receptor CD163. When the haptoglobin is saturated, the excess hemoglobin (usually now in the oxidized methemoglobin form) will release free ferriheme, which first binds to albumin, forming methemalbumin, followed by transfer to another protective plasma protein, hemopexin.
Lipophilic free heme catalyzes the formation of active radicals and intercalates into plasma membranes. The binding between heme and hemopexin allows heme to remain soluble in an aqueous environment, and the heme–hemopexin is removed by interaction with the widely expressed LDL receptor–related protein/CD91 receptor (1, 2).
Both haptoglobin and hemopexin ultimately deliver heme to the cells of the reticuloendothelial system, activating another protective process. Heme oxygenase degrades the heme to release iron, carbon monoxide, and biliverdin, and the latter is reduced to bilirubin. In this manner, the released hemoglobin is prevented from exerting damaging effects. In clinical chemistry, the hallmark of these protective systems is the unconjugated hyperbilirubinemia associated with hemolysis and even a mild/moderate increase of carboxyhemoglobin.
Author Contributions: All authors confirmed they have contributed to the intellectual content of this paper and have met the following 3 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; and (c) final approval of the published article.
Authors' Disclosures or Potential Conflicts of Interest: No authors declared any potential conflicts of interest.
- Received for publication March 29, 2012.
- Accepted for publication April 2, 2012.
- © 2012 The American Association for Clinical Chemistry