Pigments widely distributed throughout nature (e.g., heme, bile pigments, cytochromes) consisting of four pyrroles joined in a ring (porphin) structure. They are substitution products of porphin (porphyrin) and comprise several varieties, differing for the most part in the sidechains (methyl, ethyl, vinyl, formyl, carboxyethyl, carboxymethyl, etc.) present at the eight available positions on the pyrrole rings. Depending on the nature of the side chains, the prefixes dentero-, etio-, meso-, proto-, etc., are attached to p.; distribution within each class is given by type I, II, III, and IV. P. combine with various metals (iron, copper, magnesium, etc.) to form metalloporphyrins, and with nitrogenous substances.
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