ALA dehydratase !

ALA dehydratase joins two ALA into the first pyrrole ring; it is the enzyme most sensitive to lead.

Updated 2026-11-01

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Key facts

2 ALA → porphobilinogen + 2 H₂O, in the cytosol.
It is an octamer with zinc in the active site; lead displaces the zinc and inactivates it.
There is far more activity than needed: much can be lost without symptoms, except with lead or severe inherited deficiency.
Its inhibition builds up ALA (not porphobilinogen) in blood and urine.

It is not a control point of the pathway; its activity depends on available zinc.

Lead poisoning: high urine ALA with normal porphobilinogen, plus high zinc protoporphyrin.
ALA dehydratase deficiency porphyria (Doss) is autosomal recessive and very rare; it causes acute attacks like acute intermittent porphyria.
In tyrosinemia type I, succinylacetone inhibits this enzyme and causes porphyria-like neurological crises.

Think of the answer, then tap the question to see it.

Which metal does ALA dehydratase need, and which one inhibits it?

It needs zinc; lead displaces it and inhibits the enzyme.

Which metabolite rises in urine when lead blocks ALA dehydratase?

δ-Aminolevulinic acid (ALA); porphobilinogen stays normal because it is made after the block.

Which substance in tyrosinemia type I inhibits ALA dehydratase?

Succinylacetone; that is why these children can have porphyria-like neurological crises.

Educational information, not medical advice. For any symptom, see a health professional.

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Representative model, for educational purposes only.