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[Biochemistry](https://durgical.com/en/biochemistry/) · Molecule

# Porphobilinogen

Porphobilinogen is the first pyrrole ring of heme synthesis; high in urine, it points to an acute porphyria attack.

Updated 2026-11-01

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## Key facts

- It is made from two ALA (ALA dehydratase), and four of them form hydroxymethylbilane (PBG deaminase).
- It is colourless, but in urine exposed to light and air it polymerises to porphobilin, red-brown in colour.
- Very high urine porphobilinogen: the test of choice when an attack of acute intermittent porphyria, coproporphyria or variegate porphyria is suspected.
- It is normal in lead poisoning and ALA dehydratase deficiency, where only ALA rises.

**Also called**
PBG

**Formula**
C₁₀H₁₄N₂O₄

**PubChem**
[CID 1021](https://pubchem.ncbi.nlm.nih.gov/compound/1021)

## Structure

- A single pyrrole with an acetate chain, a propionate chain and an aminomethyl group, lost as NH₃ when it links to the others.

## Clinical notes

- The sample is protected from light; the result is expressed per gram of creatinine in a spot urine, so diagnosis is not delayed.
- Between attacks it may stay high in acute intermittent porphyria but fall to normal in coproporphyria and variegate porphyria.
- The classic qualitative test uses Ehrlich's reagent; today quantitative measurement is preferred.

## Pathways

- [Heme synthesis](https://durgical.com/en/biochemistry/pathways/heme-synthesis/)

## Related conditions

- [Acute intermittent porphyria](https://durgical.com/en/conditions/acute-intermittent-porphyria/)
- [Lead poisoning](https://durgical.com/en/conditions/lead-poisoning/)

## Related

- [δ-Aminolevulinic acid (ALA)](https://durgical.com/en/biochemistry/molecules/aminolevulinic-acid/)
- [ALA dehydratase](https://durgical.com/en/biochemistry/enzymes/ala-dehydratase/)
- [Porphobilinogen deaminase](https://durgical.com/en/biochemistry/enzymes/porphobilinogen-deaminase/)

## Test yourself

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

## Which test confirms an acute porphyria attack?

Markedly raised urine porphobilinogen, measured in a light-protected spot sample.

## Why does urine from a porphyria attack darken on standing?

Colourless porphobilinogen polymerises to red-brown porphobilin with light and air.

## What is porphobilinogen like in lead poisoning?

Normal: lead blocks ALA dehydratase, before porphobilinogen is formed.

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Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.

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

Sources: [Porphobilinogen (CID 1021)](https://pubchem.ncbi.nlm.nih.gov/compound/1021) (PubChem, NCBI, 2026) · Public domain; [Porphyrins urine test: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003614.htm) (MedlinePlus, US National Library of Medicine, 2026); [Porphyria: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/001208.htm) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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