Step 1 · Biochemistry

Fructose & Galactose Metabolism

Name: Fructose & Galactose Metabolism
Creator: Veli Bakalov, MD

Benign kinase blocks versus deadly downstream ones

Biochemistry·Carbohydrate Metabolism·4 min read·9 high-yield points·Reviewed by Veli Bakalov, MD @HemeOncBuddy

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Downstream-is-worse: kinase defects (fructokinase, galactokinase) are mild, while second-enzyme defects (aldolase B, GALT) trap toxic sugar-phosphates; all four are autosomal recessive.
Fructose → fructose-1-phosphate (fructokinase) → DHAP + glyceraldehyde (aldolase B), entering glycolysis below PFK-1 and bypassing its regulation.
Essential fructosuria (fructokinase deficiency) is benign: fructose in urine, no treatment needed.
Hereditary fructose intolerance (aldolase B): fructose-1-phosphate traps phosphate, blocking glycogenolysis and gluconeogenesis → hypoglycemia, vomiting, jaundice, cirrhosis when fruit, juice, or honey starts; remove fructose, sucrose, and sorbitol.
Galactokinase deficiency: galactitol from aldose reductase causes isolated infantile cataracts (failure to track or to develop a social smile).
Classic galactosemia (GALT): galactose-1-phosphate toxicity with failure to thrive, jaundice, hepatomegaly, cataracts, intellectual disability, and E. coli neonatal sepsis; remove galactose and lactose (stop breast milk, use soy formula).
Even with diet, classic galactosemia can leave speech delay and primary ovarian insufficiency.
Fructosuria and galactosemia give urine positive for reducing substances but negative on the glucose-oxidase dipstick.
Aldose reductase (NADPH) makes sorbitol; tissues lacking sorbitol dehydrogenase (Schwann cells, lens, retina, kidney) trap it, driving diabetic neuropathy, cataracts, and retinopathy.

Overview: the downstream-is-worse rule

Where the sugars come from. Dietary sucrose (glucose + fructose) and lactose (glucose + galactose) deliver fructose and galactose, which the liver funnels into glycolysis. Each sugar has two enzymes in sequence, and each has a benign KINASE-deficiency disorder and a dangerous DOWNSTREAM disorder. All four are autosomal recessive.

The more downstream the block, the more toxic the phosphorylated sugar that piles up. A kinase block leaves the free sugar, which is simply excreted or reduced; a second-enzyme block traps a sugar-phosphate inside the cell.

The four classic disorders

Disorder	Enzyme	Accumulates	Picture	Treatment
Essential fructosuria	Fructokinase	Fructose (blood, urine)	Benign, asymptomatic	None
Hereditary fructose intolerance	Aldolase B	Fructose-1-phosphate	Hypoglycemia, vomiting, jaundice, cirrhosis	No fructose, sucrose, or sorbitol
Galactokinase deficiency	Galactokinase	Galactose → galactitol	Infantile cataracts only	Limit galactose
Classic galactosemia	GALT	Galactose-1-phosphate (+ galactitol)	Failure to thrive, liver disease, cataracts, intellectual disability, E. coli sepsis	No galactose or lactose

Fructose metabolism and its disorders

Pathway. In the liver, fructokinase traps fructose as fructose-1-phosphate; aldolase B then splits it into DHAP and glyceraldehyde (which is phosphorylated to glyceraldehyde-3-phosphate). These enter glycolysis BELOW the rate-limiting PFK-1 step, so fructose bypasses glycolytic regulation and is metabolized faster than glucose.

FructoseDietary; sucrose = glucose + fructose
→ Fructose-1-phosphateFructokinase (loss = essential fructosuria)
→ DHAP + glyceraldehydeAldolase B (loss = hereditary fructose intolerance)
→ GlycolysisEnters below PFK-1, bypassing regulation

Fructose metabolism (liver)

Essential fructosuria. Fructokinase deficiency lets unphosphorylated fructose accumulate in blood and spill into the urine as a reducing sugar. It is asymptomatic and incidental, needing no treatment.

Hereditary fructose intolerance (HFI). Aldolase B deficiency causes fructose-1-phosphate to build up in the liver (and kidney). F1P sequesters inorganic phosphate, and without free phosphate glycogen phosphorylase stalls and ATP cannot be regenerated, so both glycogenolysis and gluconeogenesis fail. The result is hypoglycemia, vomiting, jaundice, and progressive liver injury leading to cirrhosis.

HFI presentation and treatment. Symptoms begin at weaning, when fruit, juice, or honey is introduced; an infant exclusively fed breast milk (sweetened with lactose, not sucrose) is spared until then. Treatment is to eliminate fructose AND sucrose AND sorbitol (which is converted to fructose). Untreated disease causes hepatic and renal failure.

Both fructosuria and galactosemia give urine that is positive for reducing substances but negative on the glucose-oxidase dipstick, because the sugar in the urine is not glucose.

Galactose metabolism and its disorders

Pathway. Galactokinase phosphorylates galactose to galactose-1-phosphate. GALT (galactose-1-phosphate uridyltransferase) then exchanges it with UDP-glucose, releasing glucose-1-phosphate (which enters glycolysis or glycogen metabolism) and UDP-galactose; UDP-galactose 4-epimerase recycles UDP-galactose back to UDP-glucose.

GalactoseDietary; lactose = glucose + galactose
→ Galactose-1-phosphateGalactokinase (loss = galactokinase deficiency)
→ Glucose-1-phosphateGALT (loss = classic galactosemia)
→ GlycolysisEpimerase interconverts UDP-galactose and UDP-glucose

Galactose metabolism

Galactokinase deficiency. A mild disorder. Unphosphorylated galactose appears in blood and urine, and excess galactose is shunted by aldose reductase to galactitol, which is trapped in the lens and osmotically damages it. The result is infantile cataracts, which may first show as failure to track objects or to develop a social smile.

Galactokinase deficiency is Kinder: cataracts are essentially the only finding.

Classic galactosemia (GALT deficiency). Toxic galactose-1-phosphate accumulates in liver, brain, and other tissues once milk feeding begins: vomiting, failure to thrive, jaundice, hepatomegaly, cataracts (from galactitol), intellectual disability, and classic E. coli neonatal sepsis, attributed to impaired neutrophil function.

A jaundiced, feeding-intolerant neonate with E. coli sepsis should trigger thoughts of classic galactosemia.

Diagnosis and treatment. Classic galactosemia is caught on newborn screening. Treatment removes galactose and lactose from the diet: stop breast milk and standard formula and use soy-based formula (galactosemia is a true contraindication to breastfeeding). Even with a strict diet, later speech delay and primary ovarian insufficiency are not fully prevented.

The sorbitol (polyol) pathway

Two steps. Aldose reductase reduces glucose to sorbitol using NADPH; sorbitol dehydrogenase then oxidizes sorbitol to fructose using NAD⁺. Tissues with both enzymes pass sorbitol on as fructose. Tissues with aldose reductase but little or no sorbitol dehydrogenase trap sorbitol, which draws in water and causes osmotic cell swelling and injury. Aldose reductase acts on galactose the same way, producing galactitol.

Who traps sorbitol

Tissue	Enzymes	Consequence in chronic hyperglycemia
Liver, ovaries, seminal vesicles	Aldose reductase + sorbitol dehydrogenase	Sorbitol converted to fructose; little damage
Lens	Mainly aldose reductase	Cataracts
Schwann cells	Aldose reductase only	Peripheral neuropathy
Retina (pericytes), kidney	Aldose reductase only	Retinopathy; renal injury

Diabetic complications. In chronic hyperglycemia, sorbitol accumulates in Schwann cells, lens, retina (pericytes), and kidney, contributing to peripheral neuropathy, cataracts, and retinopathy — the same osmotic mechanism by which galactitol causes cataracts in the galactose disorders. Consuming NADPH in the process also leaves these cells less able to regenerate glutathione.