Chapter 9 Porphyrins

Abstract

Publisher Summary Porphyrins and porphyrinogens are cyclic tetrapyrrole intermediates of the haem biosynthetic pathway, a pathway, which is also partly shared by the biosynthesis of chlorophyll and vitamin B 12 . Structurally, porphyrins are derived from porphin by substitution of the peripheral positions with various functional groups, the most common being vinyl, ethyl, methyl, acetate, and propionate groups. With the exception of protoporphyrin, the tetrapyrrole intermediates in the haem biosynthetic pathway are the porphyrinogens in which the four methine bridges of the porphyrins are reduced to methylene groups. It is at the porphyrinogen stages when enzymic modification of the side-chain acetate and propionate groups to methyl and vinyl groups, respectively, takes place. Analysis of porphyrins in blood, urine, and faeces is mainly for the diagnosis of the porphyrias, diseases due to enzyme deficiencies in the haem pathway, either inherited or acquired, causing the excessive accumulation, and excretion of porphyrins and porphyrin precursors.