Reduces the gamma-methene bridge of the open tetrapyrrole, biliverdin IX alpha, to bilirubin with the concomitant oxidation of a NADH or NADPH cofactor (PubMed:8631357, PubMed:8424666, PubMed:7929092). 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:8631357, PubMed:8424666, PubMed:7929092). NADPH, however, is the probable reactant in biological systems (PubMed:7929092). bilirubin IXalpha + NAD(+) = biliverdin IXalpha + H(+) + NADH bilirubin IXalpha + NADP(+) = biliverdin IXalpha + H(+) + NADPH Binds 1 zinc ion per subunit. Porphyrin-containing compound metabolism; protoheme degradation. Monomer. Liver. The disease is caused by variants affecting the gene represented in this entry. Belongs to the Gfo/Idh/MocA family. Biliverdin reductase subfamily.