NADPH methemoglobin reductase

NADPH methemoglobin reductase (human) HA and NH20-Et inhibit enzyme activity in mM range 55... 

Specific therapy for toxic methemoglobinemia involves the administration of methylene blue, which acts as an electron transfer agent in the NADPH-methemoglobin reductase reaction and thereby increases the activity of this system several fold (Figure 34-3). Methylene blue and sulfhemo-globin cause spectral interference in the measurement of methemoglobin with some co-oximeters but not with the Evelyn-Malloy method. ... 

The methemoglobin formed is reduced back to hemoglobin through the action of NADH-methemogtobin reductase (reaction ). A smaller fraction of the methemoglobin is reduced by a similar enzyme requiring NADPH (as indicated by the colored arrow). A hereditary lack of the NADH-methemoglobin reductase is also known. 

Yubisui T, Matsuki T, Tanishima K, Takeshita M, and Yoneyama Y (1977) NADPH-flavin reductase in human erythrocytes and the reduction of methemoglobin through flavin by the enzyme. Biochemical and Biophysical Research Communications 76, 174-82. 

Figure 34-3 Participation of methylene blue as an electron acceptor in the NADPH-dependent methemoglobin reductase reaction.

A small but constant amount of methemoglobin is produced in the body. It is reduced by specific enzymes (methemoglobin reductases) and NADH, which is generated in glycolysis. Reductases isolated from human red cells also use NADPH but to a lesser extent. Inability to reduce metHb produces methemoglobinemiaand tissue anoxia. 

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