Methaemoglobinaemia Resulting from Heterozygosity for Two NADH‐Methaemoglobin Reductase Variants: Characterization as NADH‐Ferricyanide Reductase

A case of congenital methaemoglobinaemia was found to result from heterozygosity for two different forms of NADH-methaemoglobin reductase (NADH-MR) deficiency. One of these variant forms was found to be electrophoretically fast, and to have sufficient residual activities to allow detailed characterization. This variant was characterized by its NADH-ferricyanide reductase activity, and was found to have a normal pH optimum and normal affinity for K3Fe(CN)6, NADH and deamino NADH. The diminished activity associated with this variant appeared to result from its decreased stability. It is suggested that the NADH-ferricyanide reductase activity of NADH-MR provides a convenient and standardized procedure for the characterization of NADH-MR.