• Galactosemia is an autosomal recessive disorder of galactose metabolism.
• It is caused by mutation of the gene encoding galactose-1-phosphate uridyltransferase (GALT).
• It affects about 1 in 53,000 live-born infants in the United States.
• Normally:
  • Milk contains lactose.
  • Lactase splits lactose in intestinal microvilli into:
    • glucose
    • galactose
  • Galactose is then converted toward glucose through several steps, one requiring GALT.
• In galactosemia:
  ↓ GALT → accumulation of galactose-1-phosphate + galactitol and other metabolites.
• These metabolites accumulate in:
  • liver
  • spleen
  • lens
  • kidney
  • cerebral cortex
  • red blood cells
• The liver, eyes, and brain are most severely affected.
• Liver:
  • early hepatomegaly is mainly due to fatty change.
  • later → widespread fibrosis/scarring resembling cirrhosis.
• Eye:
  • galactitol accumulates in the lens.
  • ↑ tonicity → water enters lens → lens swells → cataract.
• Brain:
  • may show loss of neurons, gliosis, and edema.
  • The exact mechanism is uncertain, but increased galactitol may contribute.
• Clinical features begin soon after milk feeding:
  • failure to thrive
  • vomiting
  • diarrhea
• During the first week:
  • jaundice
  • hepatomegaly
• Galactose and galactose-1-phosphate accumulation in the kidney → impaired amino-acid transport → aminoaciduria.
• Escherichia coli septicemia occurs with increased frequency.
• Newborn screening measures GALT enzyme activity from a dried blood spot.
• A positive screen is confirmed by measuring GALT activity in red blood cells.
• Removing galactose from the diet, especially during the first 2 years, can prevent or reduce many complications.
• If treatment begins soon after birth:
  • cataracts may be prevented
  • liver damage may be prevented
  • developmental impairment is much less severe
• Even with dietary restriction, older patients may still develop:
  • speech disorder
  • gonadal failure, especially premature ovarian failure
  • less commonly, ataxia

KEY CONCEPT

• Galactosemia = GALT deficiency → galactose-1-phosphate + galactitol accumulation.
• Major organs affected:
  liver + lens + brain.
• Classic early sequence:
  milk feeding → vomiting/diarrhea → failure to thrive → jaundice + hepatomegaly.
• Galactitol in lens → water entry → cataract.
• Early galactose restriction prevents much of the liver and eye damage.

CONCEPTUAL EXAMPLES

• Milk lactose → glucose + galactose → GALT cannot process galactose properly → toxic metabolites accumulate.
• Galactitol builds up in lens → pulls in water → cataract.
• Metabolites accumulate in liver → fatty change → later fibrosis/cirrhosis.
• Galactose removed early from diet → less toxic buildup → major complications reduced.

Lysosomal Storage Diseases

• Lysosomes are the cell’s digestive system and contain hydrolytic enzymes that break down complex substances such as:
  • sphingolipids
  • mucopolysaccharides
• These materials reach lysosomes from:
  • damaged intracellular organelles through autophagy
  • outside the cell through endocytosis or phagocytosis
• If a lysosomal enzyme is inheritedly deficient:
  substrate cannot be completely degraded → partially digested insoluble material accumulates inside lysosomes (Fig. 4.12).
• Lysosomes become enlarged and numerous → interfere with normal cell function.
• Lysosomal dysfunction also impairs autophagy → accumulation of:
  • polyubiquitinated proteins
  • dysfunctional mitochondria
• Defective mitochondria → ↑ free radicals → may trigger apoptosis.
• About 70 lysosomal storage diseases have been identified.
• They may result from defects in:
  • lysosomal enzymes
  • proteins involved in substrate degradation
  • endosomal sorting
  • lysosomal membrane integrity
• Lysosomal storage diseases are classified according to the stored substrate and accumulated metabolite (Table 4.3).
• Each disorder usually results from deficiency of a specific enzyme.
• Combined frequency is about 1 in 2500 live births.
• Lysosomal dysfunction may also be linked with more common neurologic diseases:
  • Gaucher disease carrier state → increased risk of Parkinson disease
  • Niemann-Pick type C → associated with increased risk of Alzheimer disease
• These associations reflect the many functions of lysosomes, including:
  • autophagy
  • immunity through fusion with phagosomes
  • cell membrane repair
• Common features of most lysosomal storage diseases include:
  • autosomal recessive inheritance
  • usually affect infants and young children
  • storage in mononuclear phagocytes → hepatosplenomegaly
  • frequent CNS involvement → neuronal damage
  • cell injury caused by both stored material and secondary effects such as macrophage activation and cytokine release

KEY CONCEPT

• Lysosomal enzyme defect → substrate cannot be degraded → material accumulates in lysosomes → enlarged lysosomes + impaired cell function.
• Impaired autophagy → defective mitochondria and proteins also accumulate.
• Common pattern:
  autosomal recessive + childhood onset + hepatosplenomegaly + CNS involvement.

CONCEPTUAL EXAMPLES

• Missing lysosomal enzyme → undegraded lipid remains inside lysosome → progressive intracellular storage.
• Storage in macrophages of liver and spleen → hepatosplenomegaly.
• Storage in neurons → neuronal dysfunction → CNS symptoms.
• Failed autophagy → abnormal mitochondria accumulate → free-radical injury + apoptosis.

FIG. 4.12 — LYSOSOMAL STORAGE DISEASES

🧠 Simplest idea

Lysosome = cell’s recycling/digestive bag.
If a lysosomal enzyme is missing → material cannot be completely broken down → it accumulates → lysosome swells → autophagy also fails → toxic proteins + damaged mitochondria accumulate → cell injury/death.

Whole figure in one flow

Enzyme deficiency → incomplete substrate breakdown → lysosomal storage → lysosomal dysfunction → defective autophagy → toxic material + abnormal mitochondria → free radicals → CELL DEATH1️⃣ LEFT — NORMAL AUTOPHAGY

🔵/yellow round sac = autophagosome

It surrounds old/damaged cell parts such as:

• mitochondria
• membranes
• other organelles

⬇️ Gray dotted arrow

Autophagosome + lysosome fuse

The lysosome then digests these damaged structures.

⬇️ Red arrow

Result:

Degradation + recycling of metabolites

🧠 Think:

Autophagy = cell cleans and recycles its own old parts.

2️⃣ MIDDLE LEFT — NORMAL LYSOSOMAL DEGRADATION

🟢🟡 Ball = complex substrate
= a large molecule that must be broken down step-by-step.

The lysosome contains different enzymes:

A → B → C

Each enzyme performs one step.

Complex substrate
⬇️ enzyme A
smaller intermediate
⬇️ enzyme B
smaller intermediate
⬇️ enzyme C
🟢 small diffusible end products

These small products can leave the lysosome and be reused.

Easy idea:

A, B, C = three scissors cutting one big molecule into tiny usable pieces.

3️⃣ CENTER — LYSOSOMAL ENZYME DEFICIENCY

Suppose enzyme B is missing.

The figure shows a red block at B.

So:

A works
⬇️
intermediate forms
❌ B cannot work
⬇️
breakdown stops.

Result:

The partially degraded material cannot go further.

🟢🟡 circles accumulate inside lysosomes.

⬇️

Storage of non-metabolized products

This is the basic mechanism of a:

Lysosomal storage disease

4️⃣ Why do lysosomes become enlarged?

Because more and more undigested substrate enters, but cannot leave.

So:

Enzyme missing → substrate trapped → lysosomes enlarge + multiply

Eventually this interferes with normal cell function.

5️⃣ RIGHT — LYSOSOMAL DYSFUNCTION damages AUTOPHAGY

A sick lysosome cannot properly handle material brought by autophagosomes.

Gray dotted arrow:

Lysosomal dysfunction
→ poor autophagosome processing/fusion
→ defective degradation of intracellular organelles

6️⃣ What now accumulates?

The large abnormal cell shows:

• 🟢🟡 stored metabolites
• 🟣 damaged mitochondria
• broken cellular structures
• toxic proteins

⬇️

Storage of metabolites + organelles

This is secondary storage caused by defective autophagy.

7️⃣ Damaged mitochondria → FREE RADICALS

Abnormal mitochondria stay inside the cell instead of being removed.

They generate:

Free radicals / ROS

⬇️

ROS damage:

• membranes
• proteins
• DNA

⬇️

Cell damage

⬇️

☠️ CELL DEATH

🎨 COLOR / ARROW GUIDE

• 🟢🟡 round particles = stored complex substrates/metabolites
• 🟣 structures = mitochondria/organelles
• 🟠 large sacs = lysosomes
• 🔵/yellow round sac = autophagosome
• 🔴 arrows = degradation pathway
• 🔴 block at B = missing/defective enzyme
• ⚫ arrows = products/outcomes
• Gray dotted arrows = fusion/functional relationship

⭐ Two key problems

PRIMARY STORAGE

Missing lysosomal enzyme
→ substrate cannot be degraded
→ substrate accumulates.

SECONDARY STORAGE

Lysosome stops working properly
→ autophagy fails
→ damaged organelles + toxic proteins accumulate.

🧠 Fastest exam recall

Missing enzyme → substrate storage → lysosomal dysfunction → defective autophagy → damaged mitochondria → ROS → cell death.

🔥 One-line memory

Lysosomal storage disease = “Cannot digest it → store it → lysosome fails → cell gets poisoned.”