1. Tritiated oxymesoporphyrins and their ferrihaems were tested as possible intermediates in the catabolism of haemoglobin. The tritiated compounds were injected into rats with biliary fistulae and the incorporation of the isotope into bile, bile pigment, urine, faeces, liver, kidney and spleen was measured. 2. α-Oxymesoferrihaem was extensively converted into bile pigment and specifically to the expected mesobilirubin. 3. β-Oxymesoferrihaem was poorly converted into bile pigment and was not converted into mesobiliverdin IXβ. The latter was independently shown to be excreted rapidly in bile. 4. The free oxyporphyrins were also poor precursors of bile pigment, and α-oxymesoporphyrin competed with bilirubin for excretion by the liver. 5. By analogy with the results obtained with α-oxymesoferrihaem it is concluded that α-oxyprotoferrihaem is an intermediate in the catabolism of haemoglobin, undergoing further oxidation to bile pigment under the catalysis of an enzyme of definite specificity.

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