The largest amount of heme is synthesized in the bone marrow for manufacturing hemoglobin. The liver also produces large amounts of heme, and most is used as a component of cytochromes. Some cytochromes in the liver oxidize foreign chemicals, including drugs, so that they are more easily removed from the body. Eight different enzymes drive the sequential steps in heme synthesis. These precursors, which include delta-aminolevulinic acid, porphobilinogen, and porphyrins, then appear in the blood and are excreted in the urine or stool.
Excess porphyrins cause photosensitivity, in which a person is overly sensitive to sunlight.
This occurs because when exposed to light and oxygen, porphyrins generate a charged, unstable form of oxygen that can damage the skin. Nerve damage, leading to pain and even paralysis, occurs in some porphyrias, especially when delta-aminolevulinic acid and porphobilinogen accumulate. The three most common porphyrias are porphyria cutanea tarda, acute intermittent porphyria, and erythropoietic protoporphyria. These disorders are very distinct. Their symptoms differ considerably, different tests are required for diagnosis, and different treatments are involved.