Reduces the gamma-methene bridge of the open tetrapyrrole, biliverdin IXalpha, to bilirubin with the concomitant oxidation of a NADH or NADPH cofactor (PubMed:10858451, PubMed:7929092, PubMed:8424666, PubMed:8631357). Does not reduce bilirubin IXbeta (PubMed:10858451). 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) (PubMed:7929092, PubMed:8424666, PubMed:8631357). NADPH, however, is the probable reactant in biological systems (PubMed:7929092).
