Porphyrias (pronounce) may be divided into two major types.
- Erythropoietic porphyria is a defect of porphyrin metabolism of blood-producing tissues.
- Hepatic porphyria is a defect in porphyrin metabolism of the liver.
- Either type may be hereditary (caused by a gene defect) or acquired (due,say, to poisoning).
The symptoms of porphyria cannot be completely explained by our current understanding of the biochemical defect.
- In all cases there is an identifiable abnormality of the enzymes which synthesize heme.
- This leads to accumulation of intermediates of the pathway and a deficiency of heme.
- Accumulation of porphyrinogens in the skin can lead to photosensitivity.
- The neurological symptoms cannot be explained. They are especially puzzling since the metabolic defect is confined to non-neural tissue, and no known diffusible intermediate can cause the observed neurological symptoms.
Two examples of porphyria will serve to illustrate some of the biochemical issues involved.