Porphyria (cont.)

Porphyria is a term that refers to a group of disorders -- the porphyrias -- that affect the nervous system or skin, or both. Each type of porphyria is due to the deficiency of one of the enzymes needed to make a substance in the body called heme. Enzymes are proteins that help chemical reactions happen in the body. Making heme involves a series of eight different enzymes, each acting in turn.

Heme is a red pigment composed of iron linked to a chemical called protoporphyrin. Heme has important functions in the body. The largest amounts of heme are in the blood and bone marrow in the form of hemoglobin within red blood cells. Hemoglobin gives blood its red color and carries oxygen from the lungs to all parts of the body. In the liver, heme is a component of proteins that have many functions, including breaking down hormones, drugs, and other chemicals and generating high-energy compounds that keep liver cells alive and functioning normally.

The body makes heme mainly in the bone marrow and the liver. The process of making heme is called the heme biosynthetic pathway. Each step of the process is controlled by one of eight enzymes. If any one of the enzymes is deficient, the process is disrupted. As a result, porphyrin or its precursors -- chemicals formed at earlier steps of the process -- may build up in body tissues and cause illness.

The table below lists each type of porphyria and the deficient enzyme responsible for the disorder. Porphyrias are often classified as acute or cutaneous. Acute types of porphyria affect the nervous system, whereas cutaneous types mainly affect the skin. Two forms of porphyria -- hereditary coproporphyria and variegate porphyria -- may be either acute or cutaneous, or both.

Types of Porphyria

Type of Porphyria	Deficient Enzyme
Acute Porphyrias
ALAD porphyria	delta-aminolevulinic acid dehydratase
acute intermittent porphyria	porphobilinogen deaminase
hereditary coproporphyria	coproporphyrinogen oxidase
variegate porphyria	protoporphyrinogen oxidase
Cutaneous Porphyrias
congenital erythropoietic porphyria	uroporphyrinogen III cosynthase
porphyria cutanea tarda	uroporphyrinogen decarboxylase (~50% deficiency)
hepatoerythropoietic porphyria	uroporphyrinogen decarboxylase (~90% deficiency)
hereditary coproporphyria	coproporphyrinogen oxidase
variegate porphyria	protoporphyrinogen oxidase
erythropoietic protoporphyria	ferrochelatase

The most common type of porphyria overall is porphyria cutanea tarda. In the United States, acute intermittent porphyria is the most common acute porphyria.

Source: MedicineNet.com
http://www.medicinenet.com/porphyria/article.htm