Heme is a chemical compound that contains iron and gives blood its red color. Heme is a key component of several important proteins in the body. One of the proteins is hemoglobin, which enables red blood cells to carry oxygen. Heme is also an important part of certain enzymes made by the liver. Enzymes are special proteins produced by cells and speed up chemical reactions in the body.
Heme is produced mainly in the bone marrow and liver through a complex process regulated by eight different enzymes. The enzymes work one after another in separate steps that take the starting building blocks through several different intermediate compounds (called porphyrins), finally producing heme. The porphyrins themselves are derived from compounds called porphyrin precursors. If there is a deficiency in one of these enzymes, certain porphyrins and porphyrin precursors may accumulate. These may accumulate in the
Heme precursors may also appear in excess in the blood or be excreted in the urine or stool. The accumulated heme precursors cause symptoms and can often be detected by testing to help diagnose porphyria. The specific heme precursors that accumulate and the symptoms that develop depend on which enzyme is deficient.
Porphyrias are a group of different disorders, each caused by a deficiency in one of the enzymes needed for heme production. Each enzyme deficiency is caused by an abnormality in the gene (a mutation) responsible for the production of the enzyme in question. The abnormal gene is almost always inherited from one of the parents or, rarely, both.
Porphyrias can be classified in several ways. Classification according to the specific enzyme deficiency is the most accurate.
A simpler classification system distinguishes porphyrias as acute or cutaneous:
A third classification system is based on where the excess precursors originate:
Some porphyrias are classified into more than one of these categories.
Acute porphyrias cause intermittent attacks of abdominal, mental, and neurologic symptoms. These attacks are typically triggered by prescription drugs (including oral contraceptives), alcohol, and other factors such as fasting, infections, or stress.
The most common acute porphyria is
Other acute porphyrias include
Some acute porphyrias (variegate porphyria and hereditary coproporphyria) also cause skin (cutaneous) symptoms.
Cutaneous porphyrias cause symptoms involving the skin after the skin is exposed to sunlight. In these porphyrias, certain porphyrins are deposited in the skin. When exposed to light and oxygen, these porphyrins generate an unstable form of oxygen capable of damaging the skin.
Some cutaneous porphyrias cause symptoms that tend to be continuous or intermittent. These porphyrias include
Porphyria cutanea tarda, the most common cutaneous porphyria
Congenital erythropoietic porphyria
Hepatoerythropoietic porphyria (extremely rare)
People are often unaware of the connection between their symptoms and sun exposure. Their skin becomes fragile and blistered, generally on sun-exposed areas (such as the face, neck, hands, and forearms) or on damaged skin. Identical skin symptoms are also caused by two of the acute porphyrias (variegate porphyria and hereditary coproporphyria).
Other cutaneous porphyrias cause symptoms more immediately after a sun exposure. These porphyrias are
People develop a nonblistering, burning pain within minutes or hours after sun exposure. The pain can last for hours. The skin often has no change in appearance, but swelling and redness may occur.
In order to avoid exposure to substances that can precipitate acute porphyria attacks or cutaneous porphyria symptoms, people need to know whether they carry the gene for a deficient enzyme. Commonly, the genes in a blood sample are analyzed.
A child whose parent has an enzyme deficiency that can cause an acute porphyria ideally should be tested well before puberty. The child then can learn in advance how to avoid acute attacks, which are rare before puberty.
Older family members of a person with an enzyme deficiency should also be tested to confirm or reject the possibility that they are predisposed to developing an acute porphyria or could pass the disorder to a child.
Doctors diagnose porphyrias by measuring porphyrins and porphyrin precursors in the urine. Several diseases unrelated to porphyrias, such as blood disorders, liver disorders, and exposures to toxins such as alcohol, benzene, and lead, may themselves increase the amount of urine porphyrins. This phenomenon is described as secondary porphyrinuria.