What is the pathophysiology of methemoglobinemia?

Updated: Dec 09, 2018
  • Author: Mary Denshaw-Burke, MD, FACP; Chief Editor: Emmanuel C Besa, MD  more...
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Answer

RBCs contain hemoglobin, which has a quaternary structure. Each hemoglobin molecule is composed of 4 polypeptide chains. Each of these chains is associated with a heme group, which contains iron in the reduced or ferrous form (Fe2+). In this form, iron can combine with oxygen by sharing an electron, thus forming oxyhemoglobin. When oxyhemoglobin releases oxygen to the tissues, the iron molecule is restored to its original ferrous state.

Hemoglobin can accept and transport oxygen only when the iron atom is in its ferrous form. When hemoglobin loses an electron and becomes oxidized, the iron atom is converted to the ferric state (Fe3+), resulting in the formation of methemoglobin. Methemoglobin lacks the electron that is needed to form a bond with oxygen and thus is incapable of oxygen transport.

Under normal conditions, methemoglobin levels remain below 1%; however, under conditions that cause oxidative stress, levels will rise. The low level of methemoglobin is maintained through 2 important mechanisms. The first is the hexose-monophosphate shunt pathway within the erythrocyte. Through this pathway, oxidizing agents are reduced by glutathione.


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