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 Mesobiliverdin acts as competitve inhibitor for flavin reduction, indicating that flavin and tetrapyrrole substrates compete for the same site. Monomer. Predominantly expressed in liver and erythrocytes. At lower levels in heart, lung, adrenal gland and cerebrum.