Metabolism of Galactose and Polyol Pathway

Metabolism of Galactose and Polyol Pathway

Galactose Metabolism

• The term "galactose" is derived from the Greek word "gala," meaning milk.

• Galactose is an aldohexose and the 4th epimer of glucose.

• It is primarily derived from the digestion of lactose (milk sugar), which is hydrolyzed into glucose and galactose by the enzyme lactase in the small intestine.

• Once absorbed, galactose undergoes metabolism primarily in the liver.

• Galactose is a constituent of lactose in milk and is ingested through diet.

• Galactose is not an essential nutrient because UDP glucose can form UDP galactose.

• Galactose is metabolized almost exclusively by the liver, making the galactose tolerance test a useful assessment of liver function.

• UDP galactose is the active donor of galactose during synthetic reactions.

Galactose Requirement

Galactose is required for the synthesis of the following:

• Lactose
  $UDP glucose \xrightarrow{Epimerase} UDP galactose$
  $UDP galactose + glucose \xrightarrow{Lactose synthase} Lactose$

• Glycosaminoglycans

• Cerebrosides

• Glycolipids

• Glycoproteins

Galactose Metabolism Pathway

• Step 1: Galactose is phosphorylated by galactokinase to galactose-1-phosphate.
  $Galactose + ATP \xrightarrow{Mg^{++}, Galactokinase} Galactose-1-phosphate + ADP$

• Step 2: Galactose-1-phosphate reacts with UDP-glucose, catalyzed by galactose-1-phosphate uridyl transferase (GALT), producing UDP-galactose and glucose-1-phosphate.
  $Galactose-1-phosphate + UDP-Glucose \xrightarrow{GALT} UDP-Galactose + Glucose-1-phosphate$

• Step 3: UDP-galactose is converted to UDP-glucose by UDP-galactose epimerase.
  $UDP-Galactose \xrightarrow{UDP-gal-epimerase} UDP-Glucose$

• Step 4: An alternate pathway in the liver (active after 4-5 years of life) involves galactose-1-phosphate pyrophosphorylase, which produces UDP-galactose directly from galactose-1-phosphate and UTP.
  $Galactose-1-phosphate + UTP \xrightarrow{Galactose-1-phosphate pyrophosphorylase} UDP-Galactose + PPi$

Key Enzymes

• Galactokinase: Phosphorylates galactose to galactose-1-phosphate.

• Galactose-1-phosphate Uridyl Transferase (GALT): This is the rate-limiting enzyme.

• UDP-galactose Epimerase: Catalyzes the conversion of UDP-galactose to UDP-glucose, channeling galactose into glucose metabolism.

• Galactose-1-phosphate Pyrophosphorylase: An alternate pathway that becomes active later in life.

Galactosemia

• Deficiency of galactose-1-phosphate uridyl transferase (GALT) is an inborn error of metabolism.

• The block in this enzyme leads to the accumulation of galactose-1-phosphate in the liver, which inhibits galactokinase and glycogen phosphorylase, resulting in hypoglycemia.

Inhibition of Glycogen Phosphorylase

1. Structural Resemblance: Gal-1-P resembles glucose-1-phosphate, acting as a competitive inhibitor.

2. Feedback Inhibition: High Gal-1-P signals abundant sugar-phosphates, downregulating glycogen breakdown.

3. Decreased Inorganic Phosphate (Pi): Gal-1-P trapping consumes Pi and ATP pools, reducing glycogen phosphorylase activity.

4. Secondary Liver Damage: Chronic Gal-1-P accumulation leads to hepatocellular damage and impaired enzyme function.

Other Consequences

• Reduced bilirubin uptake and conjugation lead to increased unconjugated bilirubin levels in the blood.

• Enlargement of the liver, jaundice, and severe mental retardation occur.

• Free galactose accumulates, leading to galactosemia and galactosuria.

• Galactose is reduced to dulcitol, which accumulates in the lens, causing congenital cataracts due to its osmotic effect.

• Galactose-1-phosphate may deposit in renal tubules, causing tubular damage and generalized amino aciduria.

Pathophysiology of Galactosemia

• Accumulated galactose is converted into galactitol (a sugar alcohol) by aldose reductase.

• Galactitol is osmotically active, leading to cataract formation in the lens.

• Accumulation of galactose-1-phosphate is hepatotoxic and nephrotoxic, impairing cellular function.

Diagnosis

• Congenital cataracts and the presence of galactose in blood and urine suggest galactosemia.

• Newborn screening is mandatory in many countries.

  • GALT enzyme assay (in RBCs): Detects functional activity of GALT (↓ or absent in classic galactosemia).

  • Galactose-1-phosphate level: Measures accumulated metabolite (↑↑ in blood).

  • Urine reducing substances: Detects the presence of galactose (positive but NOT glucose).

  • Genetic testing: Identifies GALT gene mutations (e.g., Q188R), confirming subtype and guiding counseling.

Treatment

• Withdrawal of lactose from the diet can recede most symptoms, but established mental retardation will not improve.

• Infants are given a lactose-free diet, which may be withdrawn after 4 years when galactose-1-phosphate pyrophosphorylase becomes active.

Galactokinase Deficiency

• A variant of the disease with milder symptoms.

Types of Galactosemia and Their Clinical Aspects

Complications (if untreated)

• Liver failure

• Renal tubular dysfunction (Fanconi-like syndrome)

• Mental retardation

• Ovarian failure (in females)

• Death (especially in classic galactosemia due to sepsis)

Polyol Pathway

• An alternative route for glucose metabolism, especially active in hyperglycemia.

• Converts glucose to sorbitol and then to fructose.

• Sorbitol is poorly absorbed from the intestine.

• Glucose is reduced to sorbitol by aldose reductase, which is then oxidized to fructose.

• This amounts to the interconversion of glucose to fructose.

Polyol Pathway Reaction

$Glucose \xrightarrow{Aldose reductase, NADPH + H^{+}} Sorbitol \xrightarrow{Sorbitol dehydrogenase, NAD^{+}} Fructose$

Key Points

• Glucose, when converted to sorbitol, cannot diffuse out of the cell easily and gets trapped there.

• Sorbitol is normally present in the lens of the eye.

• Fructose is present in semen in large quantities.

• The polyol pathway is active in the brain, and fructose is seen in CSF.

• The pathway is inactive in the liver.

Clinical Importance

• In chronic hyperglycemia (e.g., uncontrolled diabetes mellitus), the polyol pathway becomes overactive in tissues where insulin-independent glucose uptake occurs.

• The excess glucose enters the polyol pathway, and sorbitol accumulates in the lens because aldose reductase is highly active.

• Sorbitol dehydrogenase is low in the lens, causing sorbitol to be trapped inside.

• Sorbitol is osmotically active, drawing water into the lens, leading to lens fiber swelling and cataract formation.

Tissues Affected

Tissues with high aldose reductase activity and low sorbitol dehydrogenase are vulnerable:

• Lens of the eye

• Retina

• Kidney

• Schwann cells (peripheral nerves)

• Seminal vesicles

Clinical Consequences of Sorbitol Accumulation

Pathophysiological Mechanisms

• Osmotic Stress: Sorbitol is osmotically active and poorly diffusible → cell swelling, rupture.

• Oxidative Stress: Consumption of NADPH by aldose reductase reduces glutathione regeneration → increased ROS (reactive oxygen species).

Clinical Correlations

• Poor glycemic control accelerates polyol pathway activation.

• Long-term diabetic complications are partly due to polyol pathway-induced damage.

Therapeutic Insight

• Aldose reductase inhibitors (e.g., epalrestat) have been investigated for the prevention of diabetic complications like neuropathy and retinopathy.

• Still under clinical evaluation, not widely used due to side effects and limited efficacy.