Abstract
The synthesis of 18-demethyl-18-formylprotoporphyrin dimethyl ester is described and the role of this compound as an intermediate in the biosynthesis of haem a is considered. A possible relationship between a compound of this structure and the prosthetic group of myeloperoxidase is discussed. The cryptoporphyrin a fraction of ox heart extracts has been reexamined and shown to consist of a mixture of Spirographis porphyrin and its isomer which presumably arise from the in vitro oxidation of the vinyl groups in protoporphyrin.

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