Deficiency
A deficiency of porphobilinogen synthase is usually acquired (rather than hereditary) and can be caused by heavy metal poisoning, especially lead poisoning, as the enzyme is very susceptible to inhibition by heavy metals.
Hereditary insufficiency of porphobilinogen synthase is called porphobilinogen synthase (or ALA dehydratase) deficiency poprhyria. It is an extremely rare cause of porphyria, with less than 10 cases ever reported.
Lead poisoning works on the cellular level by binding to this enzyme, rendering it useless.
Read more about this topic: Porphobilinogen Synthase
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