Biochemistry · Metabolic pathway

Galactose metabolism !

Galactose from milk is turned into glucose in the liver by the Leloir pathway; if it fails, galactose builds up and harms liver, brain and lens.

Updated 2026-11-01

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Balance: ATP -1

Lactase in the small intestine splits lactose into glucose and galactose.
Leloir pathway: galactokinase → galactose 1-phosphate → GALT (with UDP-glucose) → glucose 1-phosphate + UDP-galactose; the epimerase (GALE) turns UDP-galactose back into UDP-glucose.
The epimerase also runs backwards: the body makes the UDP-galactose it needs for breast-milk lactose, glycolipids and glycoproteins; galactose is not essential in the diet.
GALT deficiency: classic galactosaemia (serious). Galactokinase deficiency: cataract, with few other symptoms.

The pathway depends mainly on how much galactose arrives; it has no major hormonal control.
When galactose accumulates, aldose reductase reduces it to galactitol, which harms the lens.

Step 1 · Lactase (intestinal β-galactosidase) irreversible Lactose + H₂O → Galactose + GlucoseAt the brush border of the small intestine it splits lactose into galactose and glucose, which are absorbed.If it fails: Little lactase: lactose reaches the colon and causes bloating, gas and diarrhoea (lactose intolerance). Lactose intolerance →
Step 2 · Galactokinase (GALK1) irreversible Galactose + ATP → Galactose 1-phosphate + ADPIt phosphorylates galactose on carbon 1 and traps it in the cell.If it fails: Its deficiency (galactosaemia type II) mainly causes cataract, because free galactose is turned into galactitol. Cataract →
Step 3 · Galactose-1-phosphate uridylyltransferase (GALT) Galactose 1-phosphate + UDP-glucose ⇌ Glucose 1-phosphate + UDP-galactoseIt swaps the UDP from UDP-glucose onto galactose 1-phosphate: glucose 1-phosphate and UDP-galactose come out, and the epimerase (GALE) turns UDP-galactose back into UDP-glucose.If it fails: Its deficiency is classic galactosaemia: galactose 1-phosphate and galactitol accumulate and damage liver, kidney, brain and lens. Classic galactosaemia →
Step 4 · Phosphoglucomutase Glucose 1-phosphate ⇌ Glucose 6-phosphateIt moves the phosphate to carbon 6: glucose 6-phosphate goes to glycolysis, to glycogen or, in the liver, to the blood as glucose.

In classic galactosaemia the milk-fed newborn develops vomiting, jaundice, hepatomegaly and risk of E. coli sepsis; cataract appears because of galactitol.
Hypolactasia (lactose intolerance) is a digestion problem, not a Leloir pathway defect: unabsorbed lactose ferments in the colon.
Newborn screening detects galactosaemia before symptoms appear.

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

Which enzyme is missing in classic galactosaemia?

Galactose-1-phosphate uridylyltransferase (GALT); galactose 1-phosphate and galactitol accumulate.

Why does cataract form when galactose builds up?

Aldose reductase turns it into galactitol, which draws water into the lens and clouds it.

Why is galactose not essential in the diet?

The epimerase (GALE) makes UDP-galactose from UDP-glucose.

Is lactose intolerance a defect of the Leloir pathway?

No: intestinal lactase is lacking, lactose is not digested and ferments in the colon; the Leloir pathway works normally.

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

Sources: Galactose catabolism (Reactome, 2026); GALT: Galactose-1-phosphate uridylyltransferase (human) (UniProt, 2026); GALK1: Galactokinase (human) (UniProt, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.