Communicated by Julius H. Comroe, Jr., July 22, 1968 In the intact organism heme and hemoglobin are converted nearly quantitatively to bilirubin. 1 2 Although several model systems have been proposed, the exact mechanism and the individual steps of this conversion remain unknown. Lemberg3 suggested a coupled oxidation with ascorbate and molecular oxygen, which in vitro leads to formation of the bile pigment precursors choleglobin and verdohemochrome; on acidification, biliverdin is obtained. This nonenzymatic system is unlikely to reflect the physiologic mechanism of heme degradation because it results in formation of a mixture of biliverdin isomers, 4 whereas in vivo only the a-isomer of the bile pigment is formed. 5 Nakajima and co-workers claimed to havecharacterized and partially purified a soluble enzyme system that converted heme to a possible precursor of biliverdin. The enzyme, which they called heme a-methenyl oxygenase, was obtained from liver and kidney homogenate; as co-factors it required NADPH, ferrous iron, and an activator extracted from liver cell nuclei by boiling water. 6 The system was unusual in its substrate specificity in that it acted only on pyridine hemochromogen, hemoglobin-haptoglobin complex, and myoglobin, 7 whereas it was inactive with hematin, oxyhemoglobin, and methemoglobin.-Moreover, spleen and bone marrow, both tissues presumably active in hemoglobin degradation, were almost devoid of enzyme activity. Subsequent reports failed to confirm the existence of a soluble heme a-methenyl oxygenase system. Levin8 and M\ur-phy and co-workers9 showed that Nakajima's findingscould be ascribed to a dialyzable andheat-stable factor of low molecular weight functioning as a reducing agent. These observations and the peculiar substrate specificity of thesys-tem argue strongly against a physiologic role for" heme a-methenyl oxygenase" in thedegradation of heme compounds.

In a preliminary communication, Wise and Drabkin10 described a light-mitochondrial system (obtained from the hemophagous organ of the dog placenta) which converted hemoglobin or heme to biliverdin; the system required NADP, NAD, oxygen, and ATP. Since this study was limited to an esoteric organ of unknown function which had long been known to contain biliverdin,'1 the rele-vance of thisobservation to the physiologic mechanism of bile pigment formation in the intactorganism was not established. Recent findings indicate that labeled hemoglobin or hemin when injected into rats is converted to bile pigment largely in the liver. 2 Furthermore, within the liver, the administered hemin is concentrated in the microsomal fraction; and endogenous hemin, formed in the liver, eventually also appears in the hepatic microsomes. 12 Finally, in its oxidized form the prosthetic group of hemoglobin is easily detached from the globin, 13 suggesting that hemoglobin-heme and unbound hemin may be degraded by the same mechanism. The microsomal fraction of the liver contains important pathways for the oxi-