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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.

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