Bilirubin, the gold within

When heme containing proteins such as hemoglobin and cytochrome P450 are degraded, their prosthetic groups cannot be metabolised and have to be excreted. Elimination of heme could be considered straightforward detoxification but a large body of evidence shows that this pathway plays a role in various physiological processes as well. The first step in the catabolism of heme is cleavage of the porphyrin ring by heme oxygenase, this reaction yields biliverdin, carbon monoxide and Iron. Two heme oxygenase (HO) isoforms mediate this process, constitutively expressed HO-2 and inducible HO-1. Many cell types express heme oxygenase, with high expression present in cells of the reticuloendothelial system involved in degradation of erythrocytes(1). The first step in heme catabolism has attracted considerable interest since heme oxygenase activity is involved in the regulation of processes such as arthrosclerosis, inflammation and diabetes1. This regulation is mediated by the products of heme oxygenase action on heme, with the potent signalling molecule carbon monoxide likely playing a major role(1)