1. Pigments widely distributed throughout nature (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 side chains (methyl, ethyl, vinyl, formyl, carboxyethyl, carboxymethyl) present at the eight available positions on the pyrrole rings. Depending on the nature of the side chains, the prefixes deutero-, etio-, meso-, proto-, etc., are attached to porphyrins. Distribution within each class is by types I, II, III, or IV. Porphyrins combine with various metals (iron, copper, magnesium) to form metalloporphyrins, and with nitrogenous substances.

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