Protoporphyrin III constitutes the prosthetic group of certain vital substances, notably hemoglobin.1 Only porphyrins of isomeric series I and III occur naturally in plants and animals, and Fischer2 pointed, out that it is inconceivable for porphyrins of types I and III to be transformed one to the other in any biologic process, that they must be formed individually in the physiologic synthesis. Porphyrin arises in the body during synthesis of hemoglobin rather than during its destruction, as had been supposed formerly, a small proportion of isomeric series I arising as a by-product of the main synthesis of isomeric series III intended for utilization in the manufacture of hemoglobin.3 The porphyrin of isomeric series I is not utilized and is excreted as coproporphyrin I, and any porphyrin of isomeric series III which has been synthesized in greater amounts than are necessary to meet the immediate requirements, or which
NESBITT S. EXCRETION OF COPROPORPHYRIN IN HEPATIC DISEASE: ISOLATION AND IDENTIFICATION OF URINARY COPROPORPHYRIN ISOMERS. Arch Intern Med (Chic). 1943;71(4):483–488. doi:10.1001/archinte.1943.00210040042005
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