porphyrin

/pawr"feuh rin/, n. Biochem.

a dark red, photosensitive pigment consisting of four pyrrole rings linked by single carbon atoms: a component of chlorophyll, heme, and vitamin B₁₂.

[1905-10; < Gk porphýr(a) PURPLE + -IN²]

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Any of a class of biologically important heterocyclic compounds of a characteristic chemical structure that includes four pyrrole groups (five-membered organic rings each containing a nitrogen atom) linked by additional carbon atoms to form a large flat ring.

As biological pigments, they and closely related molecules are responsible for many of the vivid colours in living organisms, where they often occur combined with metal ions and various substituents as coordination complexes (see compound). These include the magnesium-containing chlorophylls and the iron-containing heme group, a constituent (along with protein) of, e.g., hemoglobin, the cytochromes, and the enzyme catalase. In medicine, porphyrins are used in conjunction with light, often a laser beam, to induce reactions in the body against cancer and other diseases.

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▪ biological pigment

      any of a class of water-soluble, nitrogenous biological pigments (biochromes), derivatives of which include the hemoproteins (porphyrins combined with metals and protein). Examples of hemoproteins are the green, photosynthetic chlorophylls of higher plants; the hemoglobins in the blood of many animals; the cytochromes, enzymes that occur in minute quantities in most cells and are involved in oxidative processes; and catalase, also a widely distributed enzyme that accelerates the breakdown of hydrogen peroxide.

      Evidence indicates that, in various animals, certain porphyrins may be involved in activating hormones from the pituitary gland of the brain, including those concerned with the period of sexual heat in certain female animals. Porphyrins in the integument (skin) of some mollusks and cnidarians are regarded as being photosensitive receptors of light.

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