Disorders of Branched Amino Acid Metabolism

Question 1

How many amino acids are there?

Answer 1

21

Question 2

How many branched amino acids are there?

Answer 2

3:
Leucine
Isoleucine
Valine

Question 3

How common are branched chain amino acids (BCAA) in the human body?

Answer 3

BCAA comprise ~35% of indispensable amino acids found in muscle

Question 4

Can BCAAs be stored?

Answer 4

Excess BCAA cannot be stored and have to be removed from the body via a degradation pathway

Question 5

Why are BCAAs essential?

Answer 5

The body cannot make them, but they are critical in the building blocks of tissues

Question 6

What are the basic steps involved in the degradation of BCAAs?

Answer 6

Amino acid undergoes transamination, to form α keto-carboxylic acid

α keto-carboxylic acid Decarboxylated and acylated to form Acyl-CoA derivative

Acyl-CoA derivative is dehydrogenated and end products enter the krebs cycle

Question 7

How are amino acids transported into cells?

Answer 7

All amino acids are transported into cells by specific L-amino acid transport proteins

Question 8

What are the metabolic pathways for the different BCAAs?

Answer 8

Leucine -> Acetyl-CoA and acetoacetate (lipogenic & ketogenic)
Isoleucine -> Acetyl-CoA + Succinyl-CoA (lipogenic & glucogenic)
Valine -> Succinyl-CoA (glucogenic)

Question 9

What end products does BCAA degradation yield?

Answer 9

BCAA undergo reversible transamination inside the cell to produce branched-chain a-ketoacids (BCKA).

Specific BCKA produced are:
Leucine -> α-keto-isocaproate (KIC)
Isoleucine -> α-keto-β-methyl-valerate (KMV)
Valine -> α-ketoisovalerate (KIV).

Question 10

Where do BCAAs from muscle undergo decarboxylation?

Answer 10

Liver

Question 11

Where does degradation of BCAAs occur?

Answer 11

Degradation mainly occurs in the liver, but also occurs in the kidney, muscle, heart and adipose tissue, with different rates of transamination and decarboxylation in each tissue

Question 12

How do the rates of aminotransferase and BCKA dehydrogenase activity differ in different tissues?

Answer 12

In liver, aminotransferase activity is low. BCKAD activity is high.
In heart and muscles, aminotransferase activity exceeds BCKAD activity.

Question 13

Describe the molecular structure of BCKAD

Answer 13

Mammalian BCKAD is a member of the α-ketoacid dehydrogenase complex enzyme family
Molecular weight ~4 - 5x10^6 daltons
All α-ketoacid dehydrogenases are arranged around a core of 24 E2 sub-units.
The 24 E2 sub-units form 8 trimers, each trimer occupying the corner of a truncated “cuboid” structure

Question 14

Describe the symptoms of disorders of BCAA metabolism

Answer 14

Mental and physical retardation
Feeding problems (vomiting, lack of interest in feeding)
Lethargy
Neurological seizures
Ketoacidosis
Ketoaciduria (Characterised by “maple syrup” odour in the urine)
Clinically significant fasting hypoglycaemia

Symptoms present in neonates - possible onset just days after birth.

Some or all of these symptoms can arise - dependant on the type and severity of the metabolic defect.

Question 15

Give examples of disorders of transamination

Answer 15

Hyperleucine-isoleucinemia & Hypervalinemia reported.
Both conditions are extremely rare.
Caused by a defect in the BCAA transaminase.

Question 16

Describe the features of Maple Syrup Urine Disease

Answer 16

Disorder of the BCKAD complex

Urine has distinctive odour of maple syrup
- Odour due to presence of KMV

MSUD is an autosomal recessive metabolic disorder
Global prevalence ~1:185,000 (Based on routine screening data from 26.8 million newborns)

Levels of all 3 BCAA are greatly elevated in all sufferers
All 3 BCKA derived from BCAA’s also present urine
Milder forms of MSUD have been noted

Question 17

How is MSUD classified?

Answer 17

Genetic analysis showed multiple genetic loci involved
5 different phenotypes of MSUD have been identified
Classification is based on:
- Mode of clinical presentation
- Type of defect seen in the catalytic components of the BCKAD complex
- Biochemical response to thiamine administration

Question 18

What are the 5 different phenotypes of MSUD?

Answer 18

Classic MSUD
Intermediate MSUD
Intermittent MSUD
Thiamine-responsive MSUD
Dihydrolipoyl dehydrogenase (E3)-deficient MSUD

Question 19

What are the features of Classic MSUD?

Answer 19

Most common form with the severest symptoms
All affected siblings died as new-borns.
Levels of BCAA (particularly leucine) are elevated

Presence of alloisoleucine is diagnostic of MSUD

• Formed through racemisation of L-isoleucine or ketoenol tautomerisation (and transamination) of KMV by BCAA transaminase
• Can only occur if isoleucine and KMV levels are elevated due to BCKAD defect
• Always present in classic MSUD

Question 20

What are the features of Intermediate MSUD?

Answer 20

Only ~20 reported cases of this phenotype
No catastrophic illness in the neonatal period
Persistent elevated levels of alloisoleucine, BCAA and BCKA
Also exhibit neurological impairment.

Question 21

What are the features of Intermittent MSUD?

Answer 21

Many patients have only 5 - 20% of normal BCKAD activity
Normal early growth, development and intelligence
No early metabolic signs of disease apparent
Initial presentation of symptoms ranges between 5 months and 2 years old
- Rare cases as late as the 5th decade reported
Presentation is usually in association with infection
- Patients can suffer acute metabolic decompensation after bouts of illness or extreme stress - attacks can be severe and sometimes fatal
- Neurological symptoms
- Amino and ketoacid profiles are characteristic of MSUD during attacks

Question 22

What are the features of Thiamine-responsive MSUD?

Answer 22

Resembles intermediate MSUD.

Patients do not exhibit severe neonatal onset

Thiamine treatment reduces plasma BCAA and BCKA levels

• 10 - 1000mg/day in combination with dietary restriction of protein intake
• If treated early, little or no neurological deterioration is observed

Associated with defects in the E2 subunit of BCKAD

Question 23

What are the features of Dihydrolipoyl dehydrogenase (E3)-deficient MSUD?

Answer 23

The most rare phenotype - Only 6 patients described thus far.
E3 is a common component of several dehydrogenase complexes
- Combined deficiency of BCKA, α-ketoglutarate and pyruvate dehydrogenase complexes present
Clinically similar to intermediate MSUD
- Accompanied by severe lactic acidosis.
- Levels of lactate, pyruvate, α-ketoglutarate α-hydroxyisovalerate and α-hydroxyglutarate all increased
Levels of alanine also increased

Question 24

How is MSUD diagnosed?

Answer 24

Gene Mutations encoding catalytic sub-units can cause MSUD

Originally diagnosed using enzyme assays:

• Model reactions or enzyme reconstitution techniques
• Reliable for defects in E1 and E3, but not useful for detection of E2 activity.

Now diagnosed by Western blotting, PCR & PCR-RFLP analysis

• E2 subunit deficiency first diagnosed using western blotting
• PCR based methods can detect deletion, point mutations & expression

MSUD can be divided into 4 molecular phenotypes:
Type IA:	E1α deficiency
Type IB:	E1β deficiency
Type II:	E2 deficiency
Type III:	E3 deficiency

Question 25

How is MSUD treated?

Answer 25

All amino acid degradation disorders require dietary restriction
- Affected amino acid(s) restricted to amounts required for growth and development.
- Minimise the accumulation of damaging intermediates.
- Especially necessary for leucine
Dietary restriction must be maintained for life.

Newly diagnosed patients should have thiamine therapy trial (3 weeks at 50 - 300mg/day)

Organ transplantation (attempted)

• Liver transplantation has been attempted in patient with MSUD & liver failure
• Transplantation returned BCAA levels to ~normal with a normal protein diet.

Gene therapy (not yet)