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

# Uronic acid pathway

This side route of glucose makes UDP-glucuronic acid for detoxification and for building connective tissue. It explains why humans cannot make vitamin C.

Updated 2026-11-01

## Key facts

- Glucose 6-phosphate → glucose 1-phosphate → UDP-glucose → UDP-glucuronic acid (UDP-glucose dehydrogenase, 2 NAD⁺).
- UDP-glucuronic acid is used for conjugation (bilirubin, drugs, steroids: glucuronidation) and for glycosaminoglycans (hyaluronan, chondroitin, heparin, dermatan).
- Free glucuronate → L-gulonate (NADPH). In most animals L-gulonolactone oxidase then makes ascorbic acid; humans, other primates and guinea pigs lack it, so vitamin C is essential.
- L-gulonate → L-xylulose → xylitol → D-xylulose → D-xylulose 5-phosphate, which enters the pentose phosphate pathway.
- No ATP is produced; L-xylulose reductase deficiency causes essential pentosuria (harmless L-xylulose in urine).

## In detail

- 1. UDP-glucose dehydrogenase oxidises carbon 6 of glucose to a carboxyl group, using two NAD⁺.
- 2. UDP-glucuronosyltransferases (UGT) transfer the glucuronate to bilirubin and many drugs, making them water-soluble.
- 3. In the ER, UDP-glucuronate also gives UDP-xylose, the first sugar of proteoglycan linker chains.
- 4. Free glucuronate is reduced to L-gulonate; humans have a non-functional gene for L-gulonolactone oxidase (GULO).
- 5. L-gulonate is oxidised and decarboxylated to L-xylulose; L-xylulose reductase (gene DCXR) reduces it to xylitol, which is oxidised to D-xylulose.
- 6. Some drugs (barbiturates, chlorobutanol) increase flow through the pathway and urinary glucuronate and L-xylulose.

**Also called**
Glucuronic acid pathway, Essential pentosuria, Why humans need vitamin C

## Clinical notes

- Essential pentosuria is benign but gives a positive test for reducing sugars in urine, which can be mistaken for diabetes.
- Lack of dietary vitamin C causes scurvy because humans cannot complete this pathway to ascorbate.
- Neonates have low UGT activity, one cause of physiological jaundice; inherited UGT1A1 defects cause Gilbert and Crigler–Najjar syndromes.

## Pathways

- [Pentose phosphate pathway](https://durgical.com/en/biochemistry/pathways/pentose-phosphate-pathway/)

## Related conditions

- [Scurvy (vitamin C deficiency)](https://durgical.com/en/conditions/scurvy/)
- [Gilbert syndrome](https://durgical.com/en/conditions/gilbert-syndrome/)
- [Crigler-Najjar syndrome](https://durgical.com/en/conditions/crigler-najjar-syndrome/)

## Related

- [Glucuronidation](https://durgical.com/en/biochemistry/topics/glucuronidation/)
- [UDP-glucose](https://durgical.com/en/biochemistry/molecules/udp-glucose/)
- [Glucose 6-phosphate](https://durgical.com/en/biochemistry/molecules/glucose-6-phosphate/)
- [Pentose phosphate pathway](https://durgical.com/en/biochemistry/pathways/pentose-phosphate-pathway/)
- [Vitamin C (ascorbic acid)](https://durgical.com/en/biochemistry/molecules/vitamin-c/)
- [Glycosaminoglycans (mucopolysaccharides)](https://durgical.com/en/biochemistry/topics/glycosaminoglycans/)
- [Bilirubin](https://durgical.com/en/biochemistry/molecules/bilirubin/)
- [UGT1A1 (bilirubin UDP-glucuronosyltransferase)](https://durgical.com/en/biochemistry/enzymes/ugt1a1/)
- [NADPH](https://durgical.com/en/biochemistry/molecules/nadph/)

## Test yourself

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

## Which enzyme forms UDP-glucuronic acid and which coenzyme does it use?

UDP-glucose dehydrogenase, using two NAD⁺.

## Why must humans obtain vitamin C from the diet?

They lack a working L-gulonolactone oxidase, the last enzyme of ascorbate synthesis in the uronic acid pathway.

## What enzyme is missing in essential pentosuria, and what appears in urine?

L-xylulose reductase (DCXR); L-xylulose appears in urine, harmlessly.

## What are two uses of UDP-glucuronic acid?

Conjugation of bilirubin, drugs and steroids (glucuronidation) and synthesis of glycosaminoglycans such as hyaluronan.

## Through which intermediate does the pathway rejoin the pentose phosphate pathway?

D-xylulose 5-phosphate.

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

Sources: [Essential pentosuria: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/essential-pentosuria/) (MedlinePlus, US National Library of Medicine, 2026); [Vitamins: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002399.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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