Disease class: Metabolic diseases
Types
- porphyria, Erythropoietic
- porphyrias, Hepatic
The porphyrias are a group of rare metabolic disorders, characterised by dysfunctional biosynthesis of haem, the core protein of haemoglobin.
Etymology
The name porphyria
comes from the Greek πορφύρα (porphúra) meaning purple. This is because urine may turn
red-purple during a porphyria attack, due to accumulation of these precursor
substrates.
Pathophysiology
Haem production is the result of a long path of metabolic processes. Each step in the path is catalysed by an enzyme. The rate of conversion from substrate to product is determined by the enzyme activity. This means the rate of substrate removal is determined by enzyme activity. Here is a (very) simplified pathway:
Porphyrin precursors >1> porphyrins >2> haem
Each porphyria gene produces an enzyme with insufficient activity. This means that the substrate for that particular enzyme accumulates, since it is being produced faster than it is being removed.
If step 1 is faulty, there will be too many porphyrin precursors since their rate of conversion is too low. These accumulate in the body and they induce photosensitivity and toxic radical formation.
If step 2 is faulty, there will be normal levels of precursors, but too many porphyrins. These accumulate in the body and cause neurovisceral symptoms.
Acute porphyrias
The following two diseases feature photosensitive blistering skin lesions and/or acute neurovisceral crises.
- Variegate porphyria
- Hereditary coproporphyria
Non-acute porphyrias
In these diseases, there is an accumulation of porphyrins but not porphyrin precursors. They feature cutaneous photosensitivity and associated skin manifestations. There are no neurovisceral symptoms.- Porphyria cutanea tarda
- Erythropoietic protoporphyria
- Congenital erythropoietic porphyria
Acquired disturbances of porphyrin metabolism
- Alcohol
- Iron deficiency
- Lead poisoning
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