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:1371282, PubMed:8020496). 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:1371282, PubMed:8020496). NADPH, however, is the probable reactant in biological systems (PubMed:8020496). bilirubin IXalpha + NAD(+) = biliverdin IXalpha + H(+) + NADH bilirubin IXalpha + NADP(+) = biliverdin IXalpha + H(+) + NADPH Binds 1 zinc ion per subunit. kcat is 82.4 min(-1) at pH 6.75 (PubMed:8020496). kcat is 88.6 min(-1) at pH 8.7 (PubMed:8020496). Porphyrin-containing compound metabolism; protoheme degradation. Monomer. Belongs to the Gfo/Idh/MocA family. Biliverdin reductase subfamily.