This document provides an overview of metabolic pathways and inborn errors of metabolism. It discusses the major metabolic pathways for carbohydrates, proteins, and lipids and how genetic defects can disrupt these pathways. It also describes the three main types of inborn errors of metabolism and provides examples. The document then goes into more detail about carbohydrate metabolism, including glycolysis, pyruvate metabolism, and the tricarboxylic acid cycle. It also discusses glycogen metabolism, protein metabolism, fat metabolism, and approaches to investigating metabolic cases.

1. Metabolic Medicine
Part one
Hevi Pediatric Teaching Hospital
28-4-2013

2. Metabolic pathways
• The term metabolism refers to all biochemical
processes and pathways in the body.
• Enzymes play a key role in many of these
processes and changes in their function, as a
result, of genetic mutation can lead to
problems in these pathways.

3. • The major metabolic pathways for proteins,
carbohydrates and lipids are closely integrated
with key molecules, such as acetyl co-enzyme
A via complex mechanisms.
• A genetic defect in any part of the major
metabolic pathways is known as an inborn or
congenital (if present from birth) error of
metabolism.

4. • Inborn errors of metabolism can be divided into
three pathophysiological diagnostic groups:
• Disorders that disrupt the synthesis or
catabolism of complex molecules with
symptoms that are permanent, progressive,
independent of intercurrent events and not
related to food intake.
• These include lysosomal disorders, peroxisomal
disorders and disorders of intracellular transport
and processing.

5. • Disorders that lead to an acute or progressive
accumulation of toxic compounds as a result
of metabolic block.
• These include disorders of amino acid
metabolism , organic acidurias, congenital
urea cycle defects and sugar intolerances
(galactosaemia).

6. • Disorders with symptoms due to a deficiency
of energy production or utilisation within the
liver, myocardium, muscle or brain.
• These include congenital lactic acidemias,
fatty acid oxidation defects, gluconeogenesis
defects and mitochondrial respiratory chain
disorders.

7. BASIC METABOLISM
• Carbohydrate metabolism
• Glucose has three metabolic pathway in the
body:
• oxidation for energy,
• storage as glycogen,
• and conversion to amino acids and
triglycerides.

8. Glycolysis
• Glycolysis takes place in the cytoplasm of all
cells and describ'es the break down of one
molecule of glucose to produce two molecules
of pyruvate·
• It can occur under aerobic via the
tricarboxylic acid (TCA) cycle and oxidative
phosphorylation)
• or under anaerobic conditions via lactate.

9. • Glycolysis provides an emergency mechanism
for energy production when oxygen is limited,
i.e. in red cells (which have no mitochondria,
thus glycolysis is their only means of energy
production) or in skeletal muscle during
exercise.
• Glycolysis also provides intermediates for
other metabolic pathways, e.g. pentoses for
DNA synthesis.

10. • Pyruvate metabolism:
• Pyruvate, produced by glycolysis and other
metabolic pathways, can be converted to
• oxaloacetate (by pyruvate carboxylase) for
entry into the TCA cycle,
• or acetyi-CoA (by pyruvate dehydrogenase)

11. • Tricarboxylic acid (TCA) cycle
• This cycle is present in all cells with
mitochondria (not red cells) and provides the
final common pathway for glucose, fatty acids
and amino acid oxidation via acetyi-CoA or
other TCA cycle intermediates.
• The cycle's main function is ATP production .
• The cycle also provides metabolic
intermediates for other synthetic pathways,
e.g. amino acid synthesis.

12. • Glycogen metabolism
• Glycogen is a branched glucose polymer
stored in liver, kidney and muscle for the rapid
release of glucose when needed.
• Liver glycogen is a store to release glucose to
the rest of the body,
• whereas muscle glycogen supports muscle
glycolysis only.

13. • Glycogen synthesis is promoted by insulin.
• Glycogenolysis is promoted by adrenaline
and glucagon .

14. Protein metabolism
• Proteins are assembled from amino acids which
are composed of amino group
• Proteins metabolized to urea via ammonia.
• Proteins also metabolized a carbon skeleton
which has a number of potential metabolic fates:
• acetyi-CoA,
• pyruvate and
• ketone bodies.

15. • Amino acids may be used for protein
synthesis,
• or may be converted to other non-essential
amino" acids (transamination) or
• oxidized via the TCA cycle.
• Essential amino acids cannot be synthesized
in the body.

16. • Protein cannot be stored and therefore any
amino acids not used are catabolized,
• and hence to remain in neutral nitrogen
balance, protein is an essential constituent of
a healthy diet.

17. • Gluconeogenesis is the de novo synthesis of
glucose from non-carbohydrate sources such
as amino acids, lactate and glycerol.
• This usually occurs in the liver but also occurs
in the kidney in prolonged starvation.
• Gluconeogenesis is promoted by glucagon,
cortisol and adrenocorticotrophic hormone
(ACTH).

18. Fat metabolism
• Fat has the highest caloric value and therefore
is an essential energy source.
• Triglycerides comprise three fatty acid
molecules and one glycerol molecule which
are broken down by lipase (lipolysis).
• The released glycerol is converted to
glyceraldehyde-3-phosphate in the liver, a key
intermediate of both glycolysis and
gluconeogenesis.

19. • The fatty acids undergo ~oxidation within
mitochondria which shortens the fatty acid by
two carbons per cycle releasing acetyi-CoA for
entry to
• the TCA cycle or
• for the production of ketone bodies.

20. • Fatty acids can be synthesized from acetyi-CoA
(lipogenesis).
• Essential fatty acids cannot be synthesized by
the body
• The principal essential fatty acids are linoleic
and a-linolenic acids.

21. APPROACH TO THE METABOLIC CASE
• Inheritance
• Inborn errors of metabolism are individually rare but
collectively they have an incidence of about 1 per
3,000 to 4,000 births.
• Autosomal recessive inheritance is commonest.
• Exceptions include:
• • X-linked recessive- Lesch-Nyhan syndrome; Hunter
syndrome; ornithine transcarbamylase (OTC)
deficiency; Fabry disease; adrenoleukodystrophy
• • Autosomal dominant - Porphyrias (some recessive)
• • Matrilineal - Mitochondrial DNA mutations

22. • Presentation
• Presentation is non-specific, therefore clues
should be sought in the history.
• The commonest misdiagnosis is sepsis.

23. Clues from history
• Consanguineous parents
• Previous sudden infant death (especially late, i.e. > 6
months)
• Ethnicity (for certain conditions only)
• Previous multiple miscarriages (indicating non-viable
fetuses)
• Maternal illness during pregnancy, e.g.:
- Acute Fatty Liver of Pregnancy (AFLP) and Haemolysis,
Elevated Liver enzymes, Low Platelets (HELLP)
syndrome association with carrying fetus with long-
chain fat oxidation defect
- Increased fetal movements (in utero fits)

24. • Faddy eating (avoidance of foods that provoke
feeling unwell)
• Previous encephalopathic or tachypnoeic
episodes (latter implies acidosis)
History

25. • Inborn errors of metabolism present at times of
metabolic stress, e.g.:
• Neonatal period
• Weaning (increased oral intake, new challenges,
e.g. fructose)
• End of first year (slowing in growth rate,
therefore more protein catabolized as less used
for growth. May exceed metabolic capacity of
defective pathway)
• Intercurrent infections
• Puberty
History

26. Examination
• Clinical examination may reveal few clues in
many disorders of intermediary metabolism.
• Dysmorphic features may suggest certain
diagnoses.

27. • Peroxisomal disorders and Zellweger’s
syndrome:
• Large fontanelle, high prominent forehead,
hypoplastic supra-orbital ridges, epicanthic
folds, flat nasal bridge

28. • Pyruvate dehydrogenase deficiency:
• Epicanthic folds, flat nasal bridge, small nose
with anteverted flared alae nasi, long philtrum
Examination

29. • Cholesterol biosynthetic defects:
• Smith-Lemli-Opitz syndrome Epicanthic folds,
flat nasal bridge, syndactyly, abnormal
genitalia
Examination

30. • Lysosomal storage diseases (I cell disease):
• Hurler-like phenotype (coarse facial features,
large tongue)
Examination

31. • Odours are usually unhelpful and rarely
significant exceptions include:
Urine OdorInborn Error of Metabolism
Sweaty feetGultaric Academia
Maple syrupMaple Syrup urine disease
Boiled cabbageHypermethioninemia
Mousy or mustyPhenylketonuria
Rotten fishTrimethylaminuria
Cat urineMultiple crboxylase deficiency
Examination

32. • Eyes should be carefully examined for
• corneal clouding (mucopolysaccharidoses,
cystinosis),
• cataracts (galactosaemia, peroxisomal,
mitochondrial),
• pigmentary retinopathy (fat oxidation,
mitochondrial) and
• cherry-red spot (lay-Sachs, Niemann-Pick,
Sandhoff, GM1 ).
Examination

33. • Organomegaly is a key revealing sign.
• Hepatosplenomegaly is a feature of storage
disorders.
• Massive hepatomegaly in the absence of
splenomegaly suggests glycogen storage
disease because glycogen is not stored in the
spleen.
• More prominent splenomegaly is suggestive
of Gaucher disease.
Examination

34. Investigation
• Perform investigations at the time of
decompensation when diagnostic metabolites
are most likely to be present and avoid the
need for stress tests at a later date.

35. • Key initial metabolic investigations
• Blood gas (venous, capillary or arterial)
• Glucose
• Lactate
• Ammonia
• Amino acids (blood)
• Organic acids (urine)
• Acylcarnitines
• Ketones (urinary dipstick)
Investigation

36. Acid-base status
• Anion gap = Na + K - (CI + HC03 -)
• A normal anion gap in the presence of metabolic
acidosis signifies bicarbonate loss rather than an
excess of acid, e.g. renal or gut.
• Marked ketosis is unusual in the neonate and is
therefore highly suggestive of an underlying
metabolic disorder.
• Urea cycle defects may initially present with a
mild respiratory alkalosis because ammonia acts
directly on the brainstem as a respiratory
stimulant.
Investigation

37. Hypoglycaemia
• Hypoglycaemia is defined as a blood glucose
concentration of ~ 2.6 mmol/1, and should always be
confirmed in the laboratory.
• The key additional investigation is the presence or
absence of ketosis.
• Hypoketotic hypoglycaemia has a limited differential
diagnosis that can usually be resolved on history and
examination:
• Hyperinsulinism (endogenous or exogenous)
• Fat oxidation defects (e.g. MCAD)
• Liver failure
Investigation

38. Lactate
• Lactate is a weak acid which can be used directly
as a fuel for the brain and is readily produced
during anaerobic respiration.
• Secondary causes of lactate level anomalies (e.g.
hypoxia, sepsis, shock, liver failure, poor
sampling, etc.) are much more common than
primary metabolic causes.
• Cerebrospinal fluid (CSF) lactate is raised in
mitochondrial disorders, central nervous system
(CNS) sepsis and seizures.
Investigation

39. Ammonia
• Hyperammonaemia may result from poor
sampling (squeezed sample) and/or delays in
processing.
• The level of ammonia may give a clue to the
cause.
Differential diagnosisAmmonia concentration
(mic mol/1)
Normal< 40
Sick patient, fat oxidation defect, OA, liver failure, UCD40 to< 150
Fat oxidation defect, OA, liver failure, UCD150 to< 250
OA, liver failure, UCD250 to< 450
Liver failure, UCD450 to> 2000
OA, organic acidaemia; UCD, urea cycle defect.

40. • Transient hyperammonaemia of the newborn
(THAN) is characterized by very early onset,
• usually in the first 36 hours before feeding is
truly established.
• It is associated with low glutamine.
• THAN is managed as other urea cycle defects
but has an excellent prognosis if treated early
as the hyperammonaemia is secondary to
blood bypassing the liver (e.g. patent ductus
venosus), rather than a block in the urea cycle.
Investigation

41. • Amino acids
• Amino acids are measured in both blood and
urine. The latter reflects renal threshold, e.g.
• generalized aminoaciduria of a proximal renal
tubulopathy or the specific transporter defect
of cystinuria (Cystine, Ornithine, Arginine,
Lysine).
Investigation

42. • An increase in the serum levels of a specific
amino acid may be missed if only a urine
sample is analysed and the renal threshold
has yet to be breached.
• Plasma amino acids are useful in the work up
of a number of metabolic disorders, and are
essential in monitoring some metabolic
disorders.

43. • ↑leucine, isoleucine and valine- maple syrup
urine disease (MSUD)
• ↑ Glutamine, ↑ arginine in UCDs
• ↑ Alanine- lactic acidosis
• ↑ Glycine- non-ketotic hyperglycinaemia, OAs
• ↑ Phenylalanine- phenylketonuria
• ↑ Tyrosine- tyrosinaemia

44. Organic acids
• These are measured in urine only and are
diagnostic in many organic acidaemias, e.g.
• increased propionate in propionic acidaemia,
increased isovalerate in isovaleric acidaemia,
etc.

45. Organic acids
• Organic acids are also essential in the
diagnosis of other disorders.
• ↑ Orotic acid - UCDs, mitochondrial.
• ↑ Succinylacetone- tyrosinaemia type I
• ↑ Dicarboxylic acids- fat oxidation defects,
medium-chain triglyceride feeds,
mitochondrial.
Investigation

46. Acylcarnitines
• Carnitine conjugates with acyl-CoA intermediates
proximal to the block in fat oxidation defects.
• The chain length of the acylcarnitines formed is
diagnostic of where the block lies, e.g. medium-
chain (MCAD), very long-chain (VLCAD), etc.
• Likewise, conjugation with organic acids allows
diagnosis of organic acidaemias, e.g.
propionylcarnitine.
Investigation

47. Urate
• Urate is the end product of the breakdown of purines.
• Raised levels in plasma may indicate:
• Increased production (eg Lesch-Nyhan syndrome, GSD
type I, rhabdomyolysis) or
• Decreased excretion (familial juvenile hyperuricaemic
nephropathy, FJHN).
• It is essential to measure a concurrent urinary urate
because urate clearance in children is so efficient that
plasma levels may be in the upper normal range in
Lesch-Nyhan syndrome, whereas urinary levels are
grossly elevated.
• In FJHN the reverse is true with high plasma urate, but
low urinary urate.
Investigation

48. Acute patient screening
• Specific metabolites are used to screen acute
patients for specific disorders or groups of
disorders.
DisorderMetabolite
Peroxisomal disorders, e.g. Zellweger
syndrome, adrenoleukodystrophy
Very-long-chain fatty acids
Congenital disorders of glycosylationTransferrin isoelectric focusing
Purine disordersUrate
Smith-Lemli-Opitz syndrome7 -Dehydrocholesterol
Mucopolysaccharidoses and
mucolipidoses
Urinary glycosaminoglycans
and oligosaccharides
GalactosaemiaUrinary reducing substances
Purine and pyrimidine disordersUrinary purine and pyrimidine
metabolites

49. Secondary investigations include:
• Neuroimaging- basal ganglia signal change in
mitochondrial disorders
• Neurophysiology - mitochondrial, peroxisomal
• Echocardiogaraphy - especially hypertrophic
cardiomyopathy, mitochondrial, fat oxidation,
Pompe (GSD type II), storage disorders
• ECG - fat oxidation, mitochondrial
• EEG- metabolic encephalopathy, e.g. MSUD,
hyperammonaemia
Investigation

50. Enzymology
• Definitive diagnosis is confirmed on
enzymology.
• The sample requirement depends on which
tissues express the enzyme, e.g.
galactosaemia (blood), OTC deficiency (liver),
mitochondrial (muscle).
Investigation

51. • White cell enzymes are often requested in
patients with potential neurodegenerative or
storage disorders.
• The neurodegeneration panel includes GM1
gangliosidosis, arylsulphatase A deficiency,
Krabbe and fucosidosis in white cells.
• Where us Tay-Sachs, Sandhoff, Sly
mucopolysaccharidosis (MPS VII) and
mannosidosis in plasma;
Investigation

52. • The organomegaly panel includes GM1
gangliosidosis, Gaucher, Niemann-Pick A and
B, mannosidosis, fucosidosis and Wolman
disease in white cells.
• while Sly (MPS VII), and mannosidosis in
plasma.
Investigation

53. Acute management
• Stop feeds
• Promote anabolism- give 10% dextrose with
appropriate electrolyte additives (add insulin
rather than reduce% dextrose if
hyperglycaemiq.
• Correct biochemical disturbance along
standard guidelines, e.g. hypernatraemia, low
phosphate, etc.

54. • Clear toxic metabolites
- Dialysis- lactate, organic acids, ammonia,
leucine
- Drugs- UCDs: phenylbutyrate, sodium
benzoate; OAs: carnitine, glycine.
 Supplement enzyme cofactors- e.g. biotin,
thiamine, riboflavin

55. Specific treatment
• Dietary restriction
• Drugs, e.g. nitisinone in tyrosinaemia
• Enzyme replacement therapy in Gaucher, Fabry,
Pompe, Hurler, and Hunter syndromes
• Substrate deprivation therapy in Niemann Pick C and
Gaucher
• Transplantation (liver, bone marrow)
• Hepatocyte transfer (future)
• Gene therapy (future)
• Genetic counselling(future prenatal counselling)
• Screen siblings if indicated

56. SCREENING
• Principles of screening
• Screening for defined disorders aims to
prevent avoidable morbidity and mortality.
• The sensitivity of a screening test is the rate
of true-positives, and its specificity is the rate
of true-negatives.
• The aim is not to miss any cases with the
minimum of false-positives.

57. • The necessary requirements for including a
condition in a screening program are:
• Important health problem
• Accepted treatment
• Facilities available for diagnosis and treatment
• Latent or asymptomatic disease
• Suitable test
• Natural history understood
• Agreed case definition
• Early treatment improves prognosis
• Economic
• Case-finding may need to be continuous

58. • Neonatal blood spots are collected on day 5-8 .
• Laboratories still using the Guthrie test for PKU,
• which relies on phenylalanine-dependent
bacterial growth,
• may give false-negatives if the baby is receiving
antibiotics.
• The feeding status is also requested to ensure
adequate protein intake, but newer techniques
are able to detect PKU reliably on day 1 (routine
screening day in the USA).

59. Current UK program
• Universal
• Congenital hypothyroidism (thyroid-stimulating
hormone)
• Phenylketonuria (phenylalanine)
• Haemoglobinopathies (sickle-cell disease and
thalassaemia)
• Some regions
• MCAD
• Cystic fibrosis (to become universal)
• Galactosaemia
• Homocystinuria
• Duchenne muscular dystrophy

60. Thanks