Editorial Bilirubin, the Gold Within

When heme-containing proteins such as hemoglobin andcytochrome P450 are degraded, their prosthetic groups cannot be metabolized 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. Article see p 2556 The first step in the catabolism of heme is cleavage of the porphyrin ring by heme oxygenase (HO); this reaction yields biliverdin, carbon monoxide, and iron. Two HO isoforms mediate this process, constitutively expressed HO-2 and inducible HO-1. Many cell types express HO, with high expression present in cells of the reticuloendothelial system involved in the degradation of erythrocytes.1 The first step in heme catabolism has attracted considerable interest because HO activity is involved in the regulation of processes such a