Reduces the gamma-methene bridge of the open tetrapyrrole, biliverdin IX alpha, to bilirubin with the concomitant oxidation of a NADH or NADPH cofactor ( ). Uses the reactants NADH or NADPH depending on the pH; NADH is used at the acidic pH range (6-6.9) and NADPH at the alkaline range (8.5-8.7) ( ). NADPH, however, is the probable reactant in biological systems .
Subcellular locations: Cytoplasm, Cytosol
Liver.