<p>This entry represents biliverdin reductase A enzymes (<db_xref db="EC" dbkey="1.3.1.24"/>), reduces biliverdin (the product of haem oxygenases HO-1 and HO-2 activity) to the antioxidant bilirubin. It works by catalysing the reduction of the gamma-methene bridge of the open tetrapyrrole, biliverdin IX alpha, to bilirubin with the concomitant oxidation of a NADH or NADPH cofactor. This reaction is involved in the protohaem degradation pathway as part of porphyrin metabolism. Some enzymes are thought to use zinc as a cofactor. Human biliverdin reductase may also functions as a kinase and as a transcription factor in the MAPK (mitogen-activated protein kinase) signalling cascade [<cite idref="PUB00044616"/>].</p>