NADH-cytochrome b5 reductase 3 (CYB5R3)

NADH-cytochrome b5 reductase 3 (CYB5R3) is a critical flavoprotein enzyme that exists in two distinct isoforms: a soluble form found in erythrocytes and a membrane-bound form localized to the endoplasmic reticulum and mitochondria of other tissues (UniProt P00387). Its primary physiological role in red blood cells is the reduction of methemoglobin (Fe3+) to functional hemoglobin (Fe2+), a process vital for oxygen transport (StatPearls, 2023). In non-erythroid cells, CYB5R3 facilitates electron transfer for fatty acid desaturation, cholesterol biosynthesis, and the cytochrome P450-mediated metabolism of various drugs and xenobiotics (PubMed: 28652405). Genetic mutations in the CYB5R3 gene lead to hereditary methemoglobinemia, which can manifest as isolated cyanosis (Type I) or severe systemic neurological dysfunction (Type II) (NIH, 2021). Beyond its role in blood, CYB5R3 has been identified as a regulator of nitric oxide signaling in the vascular endothelium, suggesting its involvement in blood pressure control and cardiovascular health (PubMed: 31167900). Therapeutic interventions often involve the use of methylene blue, which acts as an electron carrier to assist the enzyme's function in acute methemoglobinemia (PubChem). Recent research also explores CYB5R3 as a target for metabolic disorders, given its role in mitochondrial function and lipid metabolism (PubMed: 28652405).

Other names

Cytochrome b5 reductase 3B5RDIA1Diaphorase-1Methemoglobin reductaseNADH-cytochrome b5 reductase

Methylene blue acts as a cofactor that is reduced to leucomethylene blue by NADPH-methemoglobin reductase; leucomethylene blue then non-enzymatically reduces methemoglobin to hemoglobin, bypassing the deficient CYB5R3 enzyme (PubChem). Ascorbic acid acts as a direct, non-enzymatic reducing agent for methemoglobin (StatPearls, 2023).