Methemoglobin Reductase (Cytochrome b₅ Reductase) Deficiency in Congenital Methemoglobinemia

Two NADH diaphorases, diaphorase I and II, were isolated from normal red cells and congenital methemoglobinemic red cells by CM-cellulose and DE 32 column chromatography. For methemoglobinemic sample, activities of diaphorase I and diaphorase II were 80% and less than 5% of those for the normal red cells, respectively. Only diaphorase II showed cytochrome b₅ reductase activity. The cytochrome b₅ reductase deficiency seems to manifest methemoglobinemia through the decrease in the enzymatic reduction of cytochrome b₅ and subsequent nonenzymatic reduction of methemoglobin by the reduced cytochrome b₅. The methemoglobinemic diaphorase II was found similar to the normal enzyme with respect to Kms for the dye and NADH, heat stability, effect of pH, and electrophoretic pattern. The ratio of the diaphorase activity to the cytochrome b₅ reductase activity was the same for both enzymes. Although the production of an abnormal enzyme molecule can not be excluded, it is possible that in this case the rate of enzyme formation is decreased.