cytochrome-b5 reductase

cy·to·chrome-b₅ re·duc·tase (siґto-krōm re-dukґtas) [EC 1.6.2.2] an enzyme of the endoplasmic reticulum and erythrocytes that catalyzes several series of redox reactions transferring electrons from NADH to an acceptor via the intermediate electron carrier cytochrome b₅. It is a flavoprotein (FAD). In the endoplasmic reticulum, the enzyme is composed of polar and hydrophobic segments and is membrane-bound; the reduced cytochrome b₅ carries electrons in several reactions of fatty acid desaturation and fatty acid elongation. In the erythrocytes, the enzyme comprises the polar segment only and is soluble; the reduced cytochrome b₅ transfers electrons to methemoglobin, reducing it to hemoglobin. Deficiency of the enzyme, an autosomal recessive trait, results in hereditary methemoglobinemia; deficiency in erythrocytes only is characterized by cyanosis whereas deficiency also in leukocytes, and sometimes brain and muscle, has been linked to both cyanosis and mental retardation. Called also NADH cytochrome-b₅ reductase, NADH methemoglobin reductase, and methemoglobin reductase (NADH).