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Gastrointestinal biology > Liver Biosynthetic Functions > Flashcards

Liver Biosynthetic Functions Flashcards

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1
Q

Describe the structure of the liver.

A
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2
Q

Discuss the function of liver: Metabolism.

A

Of protein, fats and carbs
- Glycogen synthesis (glycogenesis).
- Glycogen breakdown (glycogenolysis).
- Gluconeogenesis.
- Fatty acid metabolism (Hydrolysis of triglycerides to release glycerol and fatty acids).
- Deamination and transamination of amino acids.
- Removal of ammonia from body by synthesis of urea.

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3
Q

Describe ammonia detoxification in the liver.

A
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4
Q

Discuss the function of liver: Synthesis.

A
  • Plasma proteins (including albumin).
  • Coagulation factors.
  • Haem.
  • Lipoproteins.
  • Bile acids (primary).
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5
Q

Discuss the function of liver: Excretion and detoxification.

A
  • Bilirubin.
  • Amino acids & NH3 (Urea Cycle).
  • Cholesterol and steroid hormones.
  • Drugs.
  • Toxins.
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6
Q

How does the liver regulate metabolism fed and fasting states?

A
  • Fed: Uses ingested glucose and amino acids.
  • Fasting state: Processes nutrients to generate
    an alternative energy source.
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7
Q

Discuss Fat biosynthesis in the liver.

A
  • Most lipoproteins synthesised in liver.
  • Converting excess carbohydrates and proteins into fatty acids and triglyceride (exported and stored in adipose tissue).
  • Synthesis of large quantities of cholesterol and phospholipids …some packaged with lipoproteins and made available to rest of body.
  • Remainder excreted in bile as cholesterol or as bile acids.
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8
Q

Discuss protein biosynthesis in the liver.

A
  1. Liver tissue proteins:
    - Structural proteins
    - Enzymes
  2. Exported proteins
    - Plasma proteins (Albumin, a & b globulins and fibrinogen).
    *Hepatocytes are responsible for the synthesis of most plasma proteins and synthesis of non-essential amino acids (eg. glutamine).
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9
Q

Discuss Albumin synthesis.

A
  • Synthesised only in hepatic cells.
  • 15 g/d in healthy adult.
  • The predominant serum-binding protein.
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10
Q

Function of albumin.

A
  • Transports many substances eg. bilirubin, fatty acids, metals, ions, hormones, exogenous drugs.
  • Provides 75-80% of the oncotic pressure.
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11
Q

What is oncotic pressure?

A
  • The osmotic pressure exerted by large molecules, serves to hold water within the vascular space.
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12
Q

What is the distribution of albumin?

A
  • Enters intravascular space from hepatocytes:
    1. Enters hepatic lymphatic system and into the thoracic ducts.
    2. Passes directly into sinusoids after traversing Space of Disse (Perisinusoidal space).
  • Intravascular space:
    1. T1/2 ~17 days, degradation rate about 4% per day.
    2. Some pathological conditions increase daily loss from plasma: nephrosis, lymphoedema, ascites, oedema, intestinal lymphangioectasia.
    3. Vascular compartments of muscle, skin, liver, gut, and other tissues.
  • Extravascular spaces:
    1. All tissues, with majority being distributed in the skin.
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13
Q

What are other plasma protein synthesised by liver?

A
  • a & b globulins
  • Acute phase proteins e.g. crp.
  • Coagulation factors V, VII, IX, X, XIII, I, II.
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14
Q

Discuss a & b globulins.

A
  • Structure : glycoproteins.
  • Cleared by asialoglycoprotein receptors on surface of hepatocytes.
  • Function:
  • Variable, transport proteins, protease inhibitors, clotting factors.
  • Plasma levels can be indicative of disease
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15
Q

What causes decreased a & b globulins?

A
  • Genetic disorders (eg. a1-antitrypsin deficiency).
  • Malnutrition.
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16
Q

What causes increased a & b globulins?

A
  • Acute phase reactants: post- inflammatory or traumatic conditions.
  • Retention of high MW proteins following
    compensatory increased synthesis (eg. a2-macroglobulin)
17
Q

What are the types of acute phase proteins?

A
  • Negative acute phase proteins: transferrin.
  • Positive acute phase proteins: (CRP) & α1- antitrypsin.
    *CRP can be elevated > 1,000 fold.
18
Q

What is Haem synthesis?

A
  • A chemical structure in which a ferrous ion is chelated in the centre of a heterocyclic organic ring called a Porphyrin ring (protoporphyrin lX).
  • Porphyrins are a group of organic compounds (incl. haem).
19
Q

What is the function of haemoproteins?

A
  • Multiple haemoproteins with diverse functions
    Eg. oxygen transport, catalysis, electron transfer.
20
Q

What are the important enzymes in the synthesis of Haem.

A
  1. d- aminolevulinic acid (ALA) synthase.
  2. d- aminolevulinic acid (ALA) dehydratase.
  3. Uroporphyrinogen synthase (hydroxymethylbilane synthase / PBG deaminase) + UP-cosynthase.
21
Q

What is the function of d- aminolevulinic acid (ALA) synthase in haem synthesis?

A
  • Mitochondrial enzyme.
  • Rate limiting step in haem biosynthesis
  • Inhibited by free non-protein bound Haem by negative feedback.
  • Inducible enzyme
22
Q

What is the function of d- aminolevulinic acid (ALA) dehydratase?

A
  • Cytoplasmic enzyme.
  • Converts 2 x ALA molecules to porphobilinogen (PBG)
  • Enzyme contains sulphydryl groups so is sensitive to inhibition by lead (Pb2+) and other heavy metals (get elevated ALA-synthase and symptoms of lead poisoning).
23
Q

What is the function of Uroporphyrinogen synthase I + UP-cosynthase?

A
  • One molecule of uroporphyrinogen formed from 4 molecules of porphobilinogen (PBG).
  • Two different molecules formed: UP l and UP lll.
  • UP lll = naturally-occurring porphyrin
  • One pyrrole ring ‘flipped’ by UP-cosynthase
24
Q

Discuss the formation of haem.

A
  • Decarboxylation of acetate & propionate side groups to methyl & vinyl groups.
  • Transfer into mitochondria.
  • Chelation of iron.
25
Q

How is haem and globin synthesis regulated?

A

Haem:
- Inhibits activity of d-ALA synthase.
- Diminishes transport of d-ALA synthase from cytoplasm to mitochondria after synthesis of enzyme.
- Represses production of d-ALA synthase by regulating gene transcription.
- Stimulates globin synthesis to ensure levels of free haem remain low.
**Inhibition of d-ALA synthase and stimulation of globin synthesis is important for balancing hb production.

26
Q

What are the metabolic consequences of impaired enzyme activity?

A
27
Q

What are defects of haem synthesis?

A
  1. The Porphyrias:
    - Group of metabolic disorders caused by partial defects in one of the haem biosynthetic enzymes causing overall reductions in haem synthesis.
28
Q

What is the effect of the Porphyrias?

A
  • Accumulation of toxic intermediate products.
  • Less haem means positive feedback to boost the pathway and even more toxic intermediates formed.
  • Can be photosensitive or non-photosensitive (in skin = lesions/ blisters).
    **A complete deficiency in haem would be fatal.
29
Q

What is Acute intermittent porphyria (AIP)?

A
  • Defect in uroporphyrinogen synthase I (hydroxymethylbilane synthase/ PBG deaminase).
  • 50% reduction in enzyme activity.
  • 90% with genetic trait never present.
  • Haem synthetic pathway is terminated early, at PBG.
  • Clinical manifestations due to accumulation of ALA and PBG.
30
Q

What causes acute attacks of (AIP)?

A
  • Induction of Haem protein Cytochrome P450 synthesis:
    1. Enzyme inducing drugs (eg. barbiturates, phenytoin).
    2. Haem proteins contain >60% of haem in liver.
    3. Induction depletes small pool of free unbound haem.
    4. De-repression of d-aminolevulinic acid (ALA) synthase (loss of negative feedback to inhibit further pathway activity).
    5. ALA and PBG (substrates for UP synthase) accumulate.
31
Q

What is the treatment of AIP?

A
  • Haematin: derived from Haem (haem hydroxide) or Haemin (Hemin/Panhematin).
  • Represses d-aminolevulinic acid (ALA) synthase.
32
Q

What are the cutaneous manifestations of Porphyrias (Porphyria cutanea tarda (PCT))?

A
  • Features of PCT: Itching, erythema, blistering
  • Causes: Deficiency in uroporphyrinogen decarboxylase.
  • Consequences:
    1. Accumulation of Porphyrins
    2. Accumulation of Porphyrinogens: non-enzymatically converted to porphyrin.
    **Photo-excited by sunlight, causing cellular damage.

Gastrointestinal biology (24 decks)

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