Vitamin B₂, a component of FAD and FMN, which are coenzymes or prosthetic groups for certain enzymes (flavoproteins) that catalyze many oxidation–reduction reactions. Called also vitamin G, lactoflavin.

• r. kinase — an enzyme (a phosphotransferase) that catalyzes the conversion of free riboflavin and ATP to flavin mononucleotide (FMN) and ADP. Called also flavokinase.
• r. nutritional deficiency — causes conjunctivitis with corneal vascularization, dermatitis, glossitis and muscular weakness. Occurs rarely in dogs, but not a natural deficiency in farm mammals. In birds it causes a fall in egg production, decreased hatchability, curled toe paralysis and poor growth.