Porphyrin, Heme, and Erythrocyte Metabolism: The Porphyrias

Publisher Summary This chapter explains porphyrins, heme, and erythrocyte metabolism. The iron–porphyrin complexes are found in combination with proteins and these include hemoglobins, myoglobins, cytochromes, and catalase. The porphyrins also exist in nature in the uncombined or free state, and it is this group that is associated with the porphyrias. More recently, the development of more elegant methods of detection and identification of porphyrins, together with the use of isotopic tracer techniques, has resulted in the present understanding of the mechanisms of porphyrin biosynthesis and the biochemical bases for the disorders of porphyrin metabolism. The metabolism of the erythrocyte has also received considerable attention in recent years largely because of interest in erythrocyte preservation and survival, and to the discoveries of inherited erythrocyte enzyme deficiencies in man. The concentration of the isotopic nitrogen that was observed in heme indicated that the nitrogen atoms of glycine were direct precursors of the nitrogen atoms of the porphyrin ring. These findings were rapidly followed by extensive investigations that have almost completely elucidated the mechanisms of heme biosynthesis. In addition to contributing the nitrogen atoms, the methyl carbon atom of glycine is also incorporated into the porphyrin ring.