A FLAVOPROTEIN oxidoreductase that occurs both as a soluble enzyme and a membrane-bound enzyme due to alternative splicing of a single mRNA. The soluble form is present mainly in erythrocytes and is involved in the reduction of METHEMOGLOBIN. The membrane-bound form of the enzyme is found primarily in the endoplasmic reticulum and outer mitochondrial membrane, where it participates in the desaturation of fatty acids; cholesterol biosynthesis and drug metabolism. A deficiency in the enzyme can result in methemoglobinemia.