The final step in heme metabolism in mammals is catalyzed by the cytosolic biliverdin reductase enzymes A and B. BLVRB is found predominantly in fetal liver. BLVRA is a major component of human adult liver and is identical to the enzyme previously reported as biliverdin reductase.The FLR cDNA, which appeared to be identical to the BLVRB cDNA, encodes a deduced 206-amino acid protein. Western blot analysis of rat tissues showed highest levels of FLR in the erythrocytes and liver. Northern blot analysis detected a 1.1-kb transcript in human tissues. that flavins are the effective electron acceptor of the enzyme and purified FLR to homogeneity.