Porphyrins are chemical intermediates in the synthesis of Hb, myoglobin, and other respiratory pigments called cytochromes. They also form part of the peroxidase and catalase enzymes, which contribute to the efficiency of internal respiration. Iron is chelated within porphyrins to form heme. Heme is then incorporated into proteins to become biologically functional hemoproteins.
Tests of blood, urine, and stool are done to diagnose porphyria, an abnormal accumulation of porphyrins in body fluids. Porphyrias are a group of diseases caused by a deficit in the enzymes involved in porphyrin metabolism and abnormalities in the production of the metalloporphyrin heme. These tests are indicated in persons who have unexplained neurologic manifestations, unexplained abdominal pain, cutaneous blisters, or the presence of a relevant family history. Test results may identify clinical conditions associated with abnormal heme production, including anemia and porphyria (abnormal accumulation of the porphyrins) associated with enzyme disorders that may be genetic (hereditary) or acquired (e.g., lead poisoning, alcohol). Accumulation of porphyrins occurs in blood plasma, serum, erythrocytes, urine, and feces. A discussion of erythrocyte totals and fractionation of erythrocytes and plasma follows. For details of urine, serum, and stool testing for porphyrias, see Chapters 3, 6, and 4, respectively.
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