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Abstract. The photooxidation of tetracyclone yielding dibenzoyl stilbene was studied by means of an 18O labelling method recently used to study heme catabolism. Unlike heme degrading systems, the photooxidation of tetracyclone was found to proceed by a one‐molecule mechanism whereby both oxygen atoms in dibenzoyl stilbene are derived from the same molecule of molecular oxygen. This is evidence for a dioxobridged intermediate.

The normal pathway for haem catabolism involves the oxidation of the macrocyclic ring system at a methene-bridge carbon atom to produce biliverdin (Scheme lb). In mammals, biliverdin does not accumulate, but is enzymically reduced to the yelloworange pigment bilirubin (Scheme lc), which is excreted in the bile (Schmid, 1977). The first step in this process is probably hydroxylation at the appropriate methene bridge to produce the oxyhaem intermediate (iron-oxophorin). Tenhunen et al. (1969) have shown that haem breakdown is catalysed by the microsomal enzyme haem oxygenase. Although the stage at which haem oxygenase acts is not known with certainty, it seems likely that the initial hydroxylation step is enzymic and the subsequent reactions to produce biliverdin occur spontaneously. Haem degradation may also be studied in non-biological model systems. For example, treatment of haem in aqueous pyridine with ascorbate followed by hydrolysis readily yields biliverdin. This so-called coupled-oxidation reaction occurs also with iron complexes of synthetic porphyrins Thus octaethylhaem (Scheme Id) yields octaethylbiliverdin (Scheme le) although in this case the reaction is usually carried out in systems containing very little water. The precise nature of the ring-cleavage steps are not well understood, but they involve loss of the Fe atom, elimination of the methene carbon atom as CO and incorporation of two oxygen atoms not present in the original haem. Information on this reaction may be obtained by l8O labelling of the molecular oxygen used in the oxidation

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