[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Galactose metabolism [Biochemistry](https://durgical.com/en/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 ## Interactive map **Balance:** ATP -1 ## Key facts - 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. **Also called** Leloir pathway ## Regulation - 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. ## The steps, one by one - **Step 1 · Lactase (intestinal β-galactosidase)** irreversible Lactose + H₂O → [Galactose](https://durgical.com/en/biochemistry/molecules/galactose/) + [Glucose](https://durgical.com/en/biochemistry/molecules/glucose/) At 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 →](https://durgical.com/en/conditions/lactose-intolerance/) - **Step 2 · Galactokinase (GALK1)** irreversible [Galactose](https://durgical.com/en/biochemistry/molecules/galactose/) + [ATP](https://durgical.com/en/biochemistry/molecules/atp/) → Galactose 1-phosphate + ADP It 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 →](https://durgical.com/en/conditions/cataract/) - **Step 3 · [Galactose-1-phosphate uridylyltransferase (GALT)](https://durgical.com/en/biochemistry/enzymes/galactose-1-phosphate-uridylyltransferase/)** Galactose 1-phosphate + [UDP-glucose](https://durgical.com/en/biochemistry/molecules/udp-glucose/) ⇌ Glucose 1-phosphate + UDP-galactose It 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 →](https://durgical.com/en/conditions/classic-galactosaemia/) - **Step 4 · Phosphoglucomutase** Glucose 1-phosphate ⇌ [Glucose 6-phosphate](https://durgical.com/en/biochemistry/molecules/glucose-6-phosphate/) It moves the phosphate to carbon 6: glucose 6-phosphate goes to glycolysis, to glycogen or, in the liver, to the blood as glucose. ## Clinical notes - 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. ## Related conditions - [Classic galactosaemia](https://durgical.com/en/conditions/classic-galactosaemia/) - [Lactose intolerance](https://durgical.com/en/conditions/lactose-intolerance/) - [Cataract](https://durgical.com/en/conditions/cataract/) ## Related - [Galactose](https://durgical.com/en/biochemistry/molecules/galactose/) - [Glucose](https://durgical.com/en/biochemistry/molecules/glucose/) - [UDP-glucose](https://durgical.com/en/biochemistry/molecules/udp-glucose/) - [Galactose-1-phosphate uridylyltransferase (GALT)](https://durgical.com/en/biochemistry/enzymes/galactose-1-phosphate-uridylyltransferase/) - [Glucose 6-phosphate](https://durgical.com/en/biochemistry/molecules/glucose-6-phosphate/) - [Glycogen metabolism](https://durgical.com/en/biochemistry/pathways/glycogen-metabolism/) - [Glycolysis](https://durgical.com/en/biochemistry/pathways/glycolysis/) - [Fructose metabolism](https://durgical.com/en/biochemistry/pathways/fructose-metabolism/) ## Test yourself 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. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/biochemistry/pathways/galactose-metabolism/) Educational information, not medical advice. For any symptom, see a health professional. Sources: [Galactose catabolism](https://reactome.org/PathwayBrowser/#/R-HSA-70370) (Reactome, 2026); [GALT: Galactose-1-phosphate uridylyltransferase (human)](https://www.uniprot.org/uniprotkb/P07902/entry) (UniProt, 2026); [GALK1: Galactokinase (human)](https://www.uniprot.org/uniprotkb/P51570/entry) (UniProt, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011). [← All of biochemistry](https://durgical.com/en/biochemistry/) ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/biochemistry/pathways/galactose-metabolism/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/pathways/galactose-metabolism/