Plasma Proteins Flashcards

1
Q

What are some examples of proteins within the blood?

A
  • Albumin
  • Immunoglobulin
  • Transferrin
  • Fibrinogen
  • α1 anti-trypsin
  • Apolipoproteins
  • Complement
  • Haptoglobin
  • Caeruloplasmin
  • Prealbumin
  • Plasminogen
  • Retinol binding protein
  • β2 microglobulin
  • CRP
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2
Q

What are sources of proteins in biochemistry?

A

Secreted directly into plasma - Change in synthetic rate, secretion, clearance

  • Cell membrane proteins shed into circulation
  • Endogenous. Vs. exogenous - Microorganisms, dietary, therapeutic
  • Exocrine secretions
  • Cytoplasmic proteins
  • Transmembrane proteins
  • Organellar proteins - Cell leakage/injury
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3
Q

How are plasma proteins synthesised in the body?

A

Hepatocytes synthesise the majority of plasma proteins

  • Macrophages can also synthesise complement

Immunoglobulins synthesised by B cells

β2 microglobulin synthesised by all nucleated cells

Some post-transcriptional modification

  • Carbohydrate side chains
  • Cleavage of peptides
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4
Q

How does total plasma protein levels change?

A
  • Concentration is changed by rate of synthesis, rate of removal, elmination and volume of distribution
  • Increase in concentration of 10-20% occurs within 30 min of becoming upright
  • Change is due to increased diffusion of fluid from the vascular compartment into the interstitial compartment
  • Except when patients have been given blood or proteins intravenously, a rapid increase in total plasma protein is always due to a decrease in the volume of distribution (i.e. dehydration)
  • Total protein concentration of plasma can also fall rapidly if capillary permeability increases as protein will diffuse out into interstitial space – e.g. in patients with septicaemia or the systemic inflammatory response system
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5
Q

How do Plasma proteins travel through cells?

A
  • Half-life related to function of protein and structure
  • Some not restricted to vascular space
  • Pass through capillary walls by pinocytosis or inter-endothelial junctions ‘molecular sieve’ based on size and charge
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6
Q

How are proteins broken down?

A
  • Most taken up by pinocytosis into capillary endothelial cells or phagocytes –Some have receptor mediated intracellular recycling
  • Small proteins (<40 kDa) lost passively in glomerular filtrate. Some may be reabsorbed in renal tubules
  • Uptake by specific receptors or degradation via proteolysis
  • May require modification e.g. desialylation for degradation
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7
Q

What can lead to changes in synthesis or catabolism of proteins?

A
  • Immune response
  • Inflammation
  • Hormones
  • Medications
  • Liver disease
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8
Q

What is the function of proteins?

A
  • Transport proteins e.g. TBG, apolipoporoteins, transferrin
  • Control of extracellular fluid distribution
  • Immune function/inflammatory response
  • Clotting cascade
  • Protease inhibitors e.g. alpha 1 anti-trypsin
  • Buffering
  • Enzymes e.g. renin, coagulation proteins
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9
Q

How is transport carried out by proteins?

A
  • Albumin most important
  • Once bound analyte is usually rendered physiologically inactive
  • Changes in level of binding protein can affect plasma levels of analytes
  • Acute changes in proteins can be important in drug therapy
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10
Q

How do proteins help to control the ECF distribution?

A
  • Osmotic effect of plasma proteins produces an osmotic gradient across capillary membranes – colloid osmotic or oncotic pressure
  • Opposes outward hydrostatic pressure
  • Albumin is most important as it has low extra-vascular concentration and high intra-vascular concentration –E.g. oedema in hypoalbuminaemia
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11
Q

What are buffering roles of proteins?

A
  • Plasma proteins are an effective buffer system as amino acids can function as weak acids and/or weak bases
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12
Q

What are enzymes released into the body?

A
  • Muscle: AST, CK
  • Heart: Thrombin, CKMB, CK, AST, LDH
  • Blood cells: LDH, AST
  • Liver: ALT, AST, ALK Phos, Gamma-GT
  • Pancreas: Amylase, Lipase
  • Bone: Alk Phos
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13
Q

What causes the release of enzymes?

A
  • Intracellular enzymes appear in plasma as a consequnce of normal cell turnover
  • Increase plasma levels of intracellular enzymes due to cell damage or cell proliferation
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14
Q

How are proteins measured in from the body?

A

Nephelometry/ turbidimetry

  • Light scattering by antigen-antibody complexes
  • Use latex particles e.g. to enhance sensitivity

Electrophoresis

  • Immunofixation
  • Capillary electrophoresis
  • Iso-electric focussing
  • Densitometry

Dye-binding

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15
Q

What happens as a result of protein energy malnutrition?

A
  • In protein energy malnutrition, a source of amino acids are required for synthesis and turnover of protein. If intake < demand then breakdown of muscle
  • Protein synthesis in liver is prioritised. Non essential proteins reduced and there is increased risk of infection, and increased mortality
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16
Q

Why are proteins an ideal marker of nutritional status?

A
  • Directly reflects current protein status
  • Normally a constant catabolic rate
  • Short half-life
  • Responds only to protein or energy restriction
  • Small whole body pool

Not useful in short term

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17
Q

What are characterisitcs of albumin?

A
  • Most abundant plasma protein
  • Component of most body fluids
  • Half life 15- 19 days due to recycling
  • Synthesised in the liver
  • Preproalbumin → proalbumin → albumin
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18
Q

What causes changes in albumin?

A
  • Increased by fall in oncotic pressure
  • Decreased by cytokines (IL-6)
  • Rapid changes in disruption of endothelial function

Dependent on protein intake but not useful as marker of nutrition because of effect of inflammation and long half life

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19
Q

What are functions of albumin?

A

-Non-specific transport protein for hydrophobic substances

  • FFA, Drugs, Bilirubin, Ca, Zn etc.

-Oncotic pressure

  • Hypoalbuminaemia causes oedema
  • Use as replacement fluid to maintain intravascular volume

-Minor buffer of hydrogen ions

-Prognostic use in acute illness (APACHE

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20
Q

What leads to oedema?

A
  • Hypoalbumiaemia leads to decreased plasma oncotic pressure
  • This disturbs the equilibrium between plasma and interstitial fluid with the result that there is a decrease in the movement of the interstitial fluid back into the blood at the venular end of the capillaries.
  • The accumulation of interstitial fluid is seen clinically as oedema.
  • Relative decrease in plasma volume results in a fall in renal blood flow resulting in renin release then aldosterone causing sodium retention and therefore an increase in ECF which exacerbates oedema
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21
Q

What are causes of hypoalbuminaemia?

A

-Decreased synthesis

  • Malnutrition
  • Malabsorption
  • Liver Disease

-Increase volume of distribution

  • Overhydration
  • Increased capilary permeability through septicaemia and hypoxia

-Increase excretion/degration

  • Nephrotic Syndrome
  • Protein-losing enteropathies
  • Burns
  • Haemorrhage
  • Catabolic states (severe sepsis, fever, trauma, malignant disease)
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22
Q

What causes of albumin losses?

A

-Urine loss

  • Glomerular filtration barrier prevents losses proteins >60kDa, usually 1-2% lost but reabsorbed in PCT and degraded.
  • Small increases in urine albumin (microalbumin) are associated with early renal disease (>30 g/L)
  • Used in NICE CKD and diabetes guidelines
  • Nephrotic syndrome

-GI loss

  • Protein losing enteropathy
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23
Q

What are investigations for albumin losses?

A
  • Liver function test
  • Urine dipstick
  • Urine total protein
  • Urine microalbumin
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24
Q

What causes Hyperalbuminaemia?

A
  • Dehydration
  • Prolonged tourniquet
  • Specimen evaporation
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25
Q

What is the effect of inflammation on proteins (albumin)?

A
  • Increased capillary permeability resulting in re-distribution into the extravascular space
  • Decreased synthesis in response to IL-6
  • Change in oncotic pressure from contribution of other acute phase reactants therefore decreased synthesis
  • Change in liver priority synthesis of proteins
  • Increased catabolism
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26
Q

What are features of α1- Anti Trypsin (A1AT)?

A
  • Glycoprotein 54kDa, Major α1 globulin
  • A1AT is a member of the serine protease inhibitor family- responsible for 90% of the trypsin/elastase/protease inhibitory capacity of serum. Proteinase inhibitor (phenotype = Pi)
  • SerpinA1 of serpin superfamily are Suicide inhibitors of serine proteases. They irreversibly inhibit by binding to protease and undergoing conformational change to prevent catalysis
27
Q

How is α1- Anti Trypsin (A1AT) synthesised and catalysed?

A

Synthesis by hepatocytes

  • Positive acute phase reactant

Catabolism by pinocytosis

  • Removed by serpin-protease complex receptors (LDL-related receptor)
  • Half life 6 – 7 days

Increased in pregnancy, OCP, oestrogens

28
Q

What is used as a measure of protein-losing enteropahty?

A
  • A1AT complexed to trypsin and elastase lost in stool. Evaluation of faecal A1AT used as a measure of protein-losing enteropathy.
29
Q

Why is A1AT excess in lung protective?

A
  • Neutrophil elastase docks to methionine side-chain which cleaves the reactive centre
  • This releases A1AT from its high energy state and allows the cleaved rective loop to snap back to the other pole of the molecule (including the protease)
  • The elastase is squeezed on the other end of the molecule and is distorted – thus inactivated.
  • Suicidal and destroys both molecules
30
Q

What is the function of A1AT?

A

-Inhibit proteases

-Inhibit elastase released from neutrophils

  • A1AT deficiency associated with emphysema related to elastin degradation
  • Smoking stimulates chronic inflammation and neutrophil invasion in the lung
  • Promotes inactivation of A1AT via oxidation of its reactive site
31
Q

How does deficiency of A1AT occur?

A
  • Mutation in SERAPINA 1 gene on chromosome 14
  • Autosomal co-dominant inheritance. Normal allele is M with individuals phenotype characterised as PI MM
  • >100 deficient alleles but common deficiency alleles with clinical relevance include Z and S. >10% individuals heterozygous for variants
  • 1/3000
  • Results in liver disease in adult and neonates, or lung disease in adults i.e. emphysema, COPD
32
Q

What is the phenotype that is expressed in A1AT deficiency?

A
  • S variant single amino acid substitution Glu-Val. A1AT in these individuals can inhibit elastase as normal but slightly less A1AT is secreted into the circulation as some is broken down intracellularly.
  • The Z allele is associated with most severe symptoms as this is subject to a misfold. Loop sheet polymerisation leads to the reactive centre loops of one molecule inserted into another molecule.
33
Q

How is A1AT measured?

A

Quantitative testing for COPD before 45 years old or family history

  • Part of liver screen
  • Immunoturbidimetry/nephelometry
  • Phenotyping by electrophoresis (isoelectric focussing)
  • Genotype
  • 0.9 – 2.2 g/L (e.g. send for phenotype if <1.1 g/L). It is higher in females and elderly. Also increased in inflammatory disorders, malignancy/trauma, pregnancy, oestrogen/OCP
34
Q

How is AA1AT deficiency treated?

A
  • Not very successful. Mixed results with aerosol/nasal sprays
  • Barrier to treatment is achievement of adequate dose or circulating concentrations.
  • NICE does not recommend any specific treatment
  • Manage liver disease and COPD
  • Stop smoking!
35
Q

What is Caeruloplasmin?

A
  • α2-globulin, MW 150kDa
  • Contains 95% serum copper (6-8 Cu atoms per molecule)
  • Copper is added intracellularly to holo-caeruloplasmin by ATPase (ATP7B)
  • Catalyst for redox reactions including oxidation of iron (to allow binding to transferrin), control of membrane lipid oxidation
36
Q

How is Caeruloplasmic synthesised and what controls changed in the amounts?

A
  • Synthesised by hepatic cells and in small amounts from macrophage/lymphocytes
  • Increased in acute phase, oestrogen (pregnancy)
  • Holo-caeruloplasmin has longer half-life than apo form.
  • Measurement by immunoturbidimetry
37
Q

What leads to deficiency in Caeruloplasmin?

A

Genetic

  • Acaeruloplasminaemia. Leads to neurone degeneration, iron accumulation in the brain

Secondary deficiency

  • Failure to incorporate Cu therefore result in intracellular degradation (apo-Cp)
  • Wilsons disease (ATP7B)
  • Menkes disease (ATP7A)
38
Q

What is Wilson’s Disease?

A
  • Autosomal recessive condition where there is increased copper stores
  • Manifests 6-50 years, usually early adult (few >40 yrs)
  • Free copper deposit in liver, brain, cornea, kidney
  • Presents with liver disease (42%), Neurological (34%), haemolytic disease, renal tubular damage
39
Q

What are signs and symptoms of Wilson’s Disease?

A
  • Hepatitis – NAFLD– cirrhosis
  • Fulminant liver failure
  • Cognitive changes
  • Parkinsonian movements
  • Kayser-Fleisher rings

Heart, kidney, endocrine systems, haemolytic anaemia

40
Q

What is the investigation and results for Wilson’s Disease?

A

-Caeruloplasmin <0.2 g/L

-Serum copper may be low or within normal limits – can be elevated if fulminant liver failure

-24hr urine copper >1.6 umol/24 hr

  • Suspicious if >0.64 umol/24 hr
  • Not specific for Wilsons e.g. cholestatic disease causes block of bile flow
  • Penicillamine challenge in children (500mg, check 24 urine before and after)

-Gold standard is liver biopsy for copper

  • >4ug/g dry weight
41
Q

What is Haptoglobin?

A
  • A2 globulin that binds haemoglobin
  • Synthesised by hepatocytes
  • Three genotypes Hp 1-1 (15%, 86kDa), Hp 2-1 (50%, 86kDa + polymers), Hp 2-2 (35%, >200kDa + polymers)
  • 2α and 2 β chains are covalently linked by disulphides = haptoglobin
42
Q

What is the main function of Haptoglobin?

A

Scavenges haemoglobin in vascular space

  • On haemolysis, haemoglobin dissociates to αβ dimers which bind to haptoglobin
  • Complexes bind to CD163 receptors and are removed by the reticuloendothelial system
  • Degrade haem and proteins and recycle iron and amino acids
  • Does not bind deoxyHb or free Hb
43
Q

What can happens in intravascular haemolysis to Haptoglobin?

A
  • Concentration drops in intravascular haemolysis (half life of haptoglobin = 3-4d, half life of Hp/Hb complex 8 min)
  • Normal range 0.3 – 2.0 g/L
44
Q

What are other functions of Haptoglobin?

A
  • Antioxidant capacity
  • Inhibition on cathepsin B release from phagocytes
  • Bacteriostatic action
  • Positive APR
  • Not stimulated by haemolysis therefore can become depleted (bind to 1% of Hb in RBC) –Important in chronic haemolytic states
45
Q

What causes rise and fall in Haptoglobins?

A

Increased

  • Increased in Acute Phase Reaction (4-6 days after stimulation, Takes up to 2 weeks to fall back to normal)
  • Corticosteroids increase synthesis

Decreased

  • Rare anhaptoglobinaemia
  • Hemolytic disease and ineffective erythropoesis
  • Oestrogens
  • Newborn period
  • Malabsorption.
46
Q

What are the characteristics of Transferrin?

A
  • Plasma transport protein for iron (Fe3+)
  • Synthesised in the liver
  • Half life 8-10 days
47
Q

How does Transferrin internalise Iron?

A
  • Apotransferrin binds iron after oxidation to Fe3+
  • Bind to transferrin receptors and internalised and Fe is released from Transferrin.
  • Apotransferrin recycled back into the circulation
48
Q

What affects levels of Transferrin in the body?

A
  • Increased in iron deficiency (iron saturation decreased). May be normal or low in anaemia of chronic disease (high iron saturation)
  • Pregnancy, oestrogen
  • Negative APR (inflammation or malignancy)
  • Decreased in liver disease and malnutrition
  • Hereditary atransferrinaemia (iron overload and refractory hypochromic anaemia)
49
Q

What is Carbohydrate deficienct transferrin?

A
  • Glycosylation of transferrin may be decreased or absent in certain conditions
  • Congenital disorders of glycosylation
  • Measured by electrophoresis or isoelectric focussing
  • Occurs in CSF (B2) and can be used as a marker of CSF leakage from nose/ear
  • Increased in plasma in alcohol abuse
50
Q

What is C-Reactive Protein?

A
  • Pentraxin family (105 kDA)
  • Produced by hepatocytes in response to IL-6 and other cytokines from macrophage
  • Calcium dependent binding to lipids/polysaccharides enhance agglutination and lattice formation
  • Activates complement
51
Q

What do levels of CRP tell us?

A

Detectable within 6 hours, half-life 19 hours, Peak at 48- 72 hours

  • Normal <5 mg/L (99% <10 mg/L)
  • 10 – 40 mg/L mild inflammation
  • 40 – 200 mg/L significant acute bacterial infection
  • >200 mg/L burns, serious infection
52
Q

How is CRP/hs-CRP catabolised and levels altered?

A
  • Catabolised by phagocytes
  • Generally proportional to tissue damage but remember is non specific therefore need clinical information for interpretation
53
Q

What are risks of increased CRP?

A

Risk of CV disease

  • ?due to chronic intimal inflammation
  • High sensitivity assays required
54
Q

What is Procalcitonin?

A
  • Procalcitonin is made during the process of producing the thyroid hormone calcitonin
  • Made by C cells and present in low levels in the blood
  • Can also be made by other cells in the body when stimulated by certain disease processes e.g. sepsis
  • Levels of procalcitonin in the blood increase rapidly and to high concentrations when a person has sepsis.
55
Q

What are causes of increase Procalcitonin?

A

Increased procalcitonin

  • Infection from other causes
  • Tissue damage due to events such as trauma, surgery, burns, heart attack;
  • Rapid and severe organ (kidney, heart, lung etc) transplant rejection.
56
Q

How does infection control Procaltonin?

A
  • Difference in stimulus gives procalcitonin the potential to be used to help detect severe bacterial infection at an early stage and to be able to distinguish between a bacterial infection and another cause of a serious illness.
  • They are not as particularly increased when a person has a viral infection or other illnesses which may have the same or similar symptoms as sepsis
57
Q

What are the inflammatory response mediators for acute phase proteins?

A
  • Complement: Opsonisation, chemotaxis, mast cell degranulation
  • CRP: Complement activation, opsonisation
  • Factor VIII: Fibrin matrix, Vascular permeability
  • Plasminogen: Complement activation, clotting, fibrinolysis
58
Q

Which proteins are useful in Acute Phase Proteins?

A

Proteinase inhibition

  • A1AT
  • Haptoglobin

Immune regulation

  • CRP

Scavengers

  • Caeruloplasmin
  • CRP
  • Haptoglobin

Repair and resolution

  • A1AT
  • A1 chymotrypsin
59
Q

What are proteins with positive Acute Phase Response?

A
  • CRP
  • Α1 anti trypsin
  • Complement
  • Ferritin
  • Haptoglobin
  • Fibrinogen
  • C1 inhibitor
  • Plasminogen
  • Caeruloplasmin
  • Procalcitonin
60
Q

What are proteins with negative Acute Phase Responses?

A
  • Albumin
  • Apolipoprotein
  • IGF-1
  • Prealbumin
  • Retinol binding protein
  • Thyroxine binding protein
  • Transferrin
61
Q

What are other effects of the Acute Phase Response?

A

-Neuroendocrine

  • Fever, lethargy, anorexia
  • Increased cortsiol (CRH, ACTH)
  • Increased ADH
  • Decreased IGF-1
  • Increased catecholamines

-Haematopoetic

  • Anaemia of chronic disease
  • Leukocytosis
  • Thrombocytosis

-Metabolic

  • Loss of muscle, negative nitrogen balance
  • Decreased gluconeogenesis
  • Bone resorption
  • Increased hepatic lipogenesis/ lipolysis in adipose tissue
  • Cachexia

-Hepatic

-Increased metallothionein, NO synthase, haem oxygenase

-Hypozincaemia, hypoferraemia, hypercupraemia

62
Q

What is the Acute Phase Response?

A
  • Systemic inflammation in response to infection, tissue injury, or inflammatory disease induces changes in hepatic production of plasma proteins
  • Mediated by IL-6 and other cytokines which are Dependent on insult
  • Negative acute phase proteins and Positive acute phase proteins
  • Acute phase proteins are defined as those proteins whose serum concentrations increase or decrease by at least 25 percent during inflammatory states
63
Q

What is ESR?

A

The erythrocyte sedimentation rate (ESR), an indirect APR, reflects plasma viscosity and the presence of acute phase proteins, especially fibrinogen, as well as other influences, some of which are as yet unidentified