Broad specificity oxidoreductase that catalyzes the NADPH-dependent reduction of a variety of flavins, such as riboflavin, FAD or FMN, biliverdins, methemoglobin and PQQ (pyrroloquinoline quinone). Contributes to heme catabolism and metabolizes linear tetrapyrroles. Can also reduce the complexed Fe(3+) iron to Fe(2+) in the presence of FMN and NADPH. In the liver, converts biliverdin to bilirubin. NADP(+) + reduced riboflavin = 2 H(+) + NADPH + riboflavin bilirubin IXalpha + NAD(+) = biliverdin IXalpha + H(+) + NADH bilirubin IXalpha + NADP(+) = biliverdin IXalpha + H(+) + NADPH Monomer. At least expressed in the liver and erythrocyte.