Reactions between uroporphyrin and copper and their biological significance

Copper-porphyrin occurs naturally only as the red feather pigment turacin (copper-uroporphyrin III) which has a characteristic absorption spectrum, does not combine with ligands and is catalytically inactive. The formation of copper-uroporphyrin in vitro was studied using uroporphyrin I. This combined readily with inorganic copper within the physiological range of pH and temperature; the formation of the copper-porphyrin, which has the same properties as turacin, was followed spectro-photometrically and manometrically by the inhibitory action of the uroporphyrin on the catalytic activity of copper in the oxidation of ascorbate. The copper-uroporphyrin formed under the latter conditions is oxidised by H 2O 2 liberated in the system to a greenish pigment but may be protected from this oxidative destruction on by catalase. The oxidation of the copper-porphyrin was studied spectrophoto-metrically. Uroporphyrin did not react with the copper prosthetic group of caerulo-plasmin nor with the non-caeruloplasmin fraction of plasma copper, i.e. the fraction which reacts directly sodium diethyldithicarbamate. Plasma proteins, especially albumin, had some inhibitory action on the combination of uroporphyrin with added copper. This was i nvestigated for pig chicken plasmas. The possible biological significance of these results is discussed with special reference to congenital porphyria in man, cattle and pigs and the formation of turacin in vivo.