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. 2021 Feb 3;22(4):1514. doi: 10.3390/ijms22041514

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© 2021 by the author.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Proposed mechanism of MetHb accumulation in blood. Under normal conditions, small amounts of Hb derived from hemolysis are rapidly bound by circulating Hp, forming Hb-Hp complexes, which are rapidly taken up by the liver through CD163 (A). When massive hemolysis occurs, as in the case of hemolytic anemia, large amounts of free Hb are bound by serum Hp and transferred to the liver (B). This results in a significant reduction of serum Hp and increased indirect bilirubin (Bil) derived from the heme degradation pathway. In HO-1-deficient patients, both OxyHb and MetHb bound to Hp accumulate within the serum. Despite the apparent intravascular hemolysis, total Hp content is significantly increased, serum bilirubin level remains low, and overflow of Hb-Hp complex into the urine is seen (C).