Plasma Proteins and Their Clinical Importance

Plasma Proteins and Their Clinical Importance

Learning Outcomes

• List various plasma proteins.
• Understand the clinical importance of various plasma proteins.

Case Study: Alpha 1 Antitrypsin Deficiency

• A family exhibits a severe obstructive lung disease leading to emphysema.
• Blood analysis reveals abnormally low concentrations of alpha 1 antitrypsin.
• Understanding the basis of this condition is crucial.

Introduction to Plasma

• Total blood volume in an adult human being is about 4.5 to 5 liters.
• Plasma constitutes about 55–60% of blood.
• It is obtained by centrifuging blood mixed with an anticoagulant.

Serum vs. Plasma

• Serum is obtained from blood that is allowed to clot without an anticoagulant.
• It lacks coagulation factors, including prothrombin and fibrinogen.

Plasma Protein Composition

• Plasma proteins consist of:
  • Albumin: 3.5 to 5 g/dL
  • Globulins: 2.5 – 3.5 g/dL
  • Fibrinogen: 200–400 mg/dL
• Total protein: 6 to 8 gram/dL
• Albumin: globulin ratio is usually between 1.2:1 to 1.5:1.

Synthesis of Plasma Proteins

• Almost all plasma proteins, except immunoglobulins, are synthesized in the liver.
• Plasma proteins are generally synthesized on membrane-bound polyribosomes.
• Most plasma proteins are glycoproteins.

Electrophoresis

• Electrophoresis refers to the movement of charged particles through an electrolyte when subjected to an electric field.

Agar Gel Electrophoresis

• Normal serum separates into 5 bands:
  • Albumin: 55–65%
  • Alpha-1 globulin: 2–4%
  • Alpha-2 globulin: 6–12%
  • Beta globulin: 8–12%
  • Gamma globulin: 12–22%

Abnormal Electrophoretic Patterns in Clinical Diseases

• Chronic infections: Gamma globulins are increased (smooth and wide-based).
• Multiple myeloma: Sharp spike (M-band) due to monoclonal origin of immunoglobulins.
• Fibrinogen: In plasma electrophoresis, fibrinogen forms a prominent band in the gamma region, which may be confused with the M-band.
• Primary immune deficiency: Gamma globulin fraction is reduced.

Specific Disease Patterns

• Nephrotic syndrome: Loss of most proteins except macroglobulin; prominent alpha-2 fraction.
• Cirrhosis of liver: Decreased albumin synthesis with compensatory globulin synthesis; thin albumin band, wide beta fraction, sometimes fused with gamma.
• Chronic lymphatic leukemia: Gamma globulin fraction is reduced.
• Alpha-1 antitrypsin deficiency: Thin or missing alpha-1 band.

Electrophoresis Patterns

• Normal pattern shows distinct bands for albumin, alpha-1, alpha-2, beta, and gamma globulins.
• Nephrotic syndrome: hypo-albuminemia, prominent alpha-2 band.
• Cirrhosis: hypo-albuminemia with beta-gamma bridging.
• Chronic infection: broad based increase in gamma region; general increase in alpha-1 and alpha-2 bands, comparative reduction of albumin.
• Multiple myeloma: monoclonal band (M-band) between beta and gamma.
• Acute Inflammation: reduced albumin and increased alpha-2 fraction.

Albumin

• Albumin constitutes the major part of plasma proteins.
• It has one polypeptide chain with 585 amino acids.
• Molecular weight of 69,000 D; elliptical shape.
• Synthesized by hepatocytes; estimation of albumin is a liver function test.
• Present in CSF and interstitial fluid.
• Half-life of albumin is about 20 days.
• Liver produces about 12 g of albumin per day, representing about 25% of total hepatic protein synthesis.

Functions of Albumin

• Colloid Osmotic Pressure of Plasma:
  • Reduced protein concentration decreases effective osmotic pressure (EOP), diminishing water return into blood vessels, leading to edema.
• Transport Function:
  • Carrier of various hydrophobic substances in the blood.
    • Bilirubin and non-esterified fatty acids are specifically transported by albumin.
    • Drugs (sulfa, aspirin, salicylate, dicoumarol, phenytoin).
    • Hormones: Steroid hormones, thyroxine.
    • Metals: Albumin transports copper. Calcium and heavy metals are non-specifically carried by albumin.
  • Only the unbound fraction of drugs is biologically active.
• Buffering Action:
  • Albumin has 16 histidine residues contributing to buffering action.
• Nutritional Function:
  • Tissue cells take up albumin by pinocytosis, breaking it down to amino acids.
  • Albumin may be considered the transport form of essential amino acids from liver to extrahepatic cells.

Clinical Applications of Albumin

• Blood Brain Barrier:
  • Albumin-fatty acid complex cannot cross the blood-brain barrier.
  • Unconjugated bilirubin, if displaced by drugs like aspirin, can cross the blood-brain barrier, leading to kernicterus and mental retardation in young children.

Jaundice and Kernicterus

• Excess bilirubin in the blood leads to yellowing of the skin and eyes (jaundice).
• Bilirubin moving from the bloodstream into brain tissue causes kernicterus.

Drug Interactions

• Competition for albumin binding sites can occur when two drugs with high affinity are administered together, leading to displacement of one drug.

Protein-Bound Calcium

• Calcium level in blood is lowered in hypoalbuminemia.
• Ionized calcium level may be normal despite lowered total calcium, so tetany may not occur.

Therapeutic Use of Albumin

• Human albumin is therapeutically useful to treat burns, hemorrhage, and shock.

Role of Albumin in Burns

• Severe burns cause massive plasma loss, leading to hypovolemia and edema.
• Albumin therapy helps by:
  • Restoring plasma volume, preventing hypovolemic shock.
  • Maintaining oncotic pressure, reducing fluid leakage from blood vessels.
  • Enhancing tissue perfusion, improving oxygen and nutrient delivery.
• Clinical Use: Albumin is often used after initial fluid resuscitation with crystalloids.

Role of Albumin in Hemorrhage

• Major blood loss leads to hypovolemia, decreased oxygen delivery, and organ dysfunction.
• Albumin therapy helps by:
  • Acting as a plasma expander, rapidly increasing intravascular volume.
  • Preventing shock by maintaining blood pressure and perfusion.
  • Supporting organ function by improving circulation.
• Clinical Use: Albumin is used alongside blood transfusions when crystalloids alone are insufficient to restore volume.

Colloids Vs Crystalloids

• Colloids:
  • Includes Albumin, Dextran, Hydroxyethyl starches (HES), Gelatin
  • Large molecules that stay in intravascular space longer
  • Fast at expanding intravascular space & amount administered equal to amount lost
  • Risks: allergic reaction, coagulation problems
• Crystalloids:
  • Includes Hypotonic, Hypertonic, Isotonic solutions
  • Small molecules that don't stay too long in intravascular space
  • High amount of fluids needed to equal amount lost (overload: edema)
  • No allergic reactions or coagulation problems
• Types of I.V. Fluids:
  • Crystalloids: Normal (0.9%) saline, Ringer's lactate solution (Hartmann's solution), 5% Dextrose
  • Colloids: Human Albumin, Gelation Solutions (Haemaccel, Gelafundin), Dextran, Hydroxyethyl starches (Hetastarch)
  • Cost: Colloids more, Crystalloids Less and easier to access

Role of Albumin in Shock

• Shock is a state of circulatory failure leading to inadequate oxygen delivery to tissues.
• Albumin therapy helps by:
  • Increasing oncotic pressure, pulling fluid back into circulation.
  • Improving cardiac output and perfusion.
  • Enhancing microcirculatory flow, reducing organ dysfunction.
• Clinical Use: Albumin is often used in septic and hypovolemic shock when patients do not respond well to crystalloids.

Normal Albumin Levels

• Normal level of Albumin is 3.5–5 g/dL.

Hypoalbuminemia

• Causes:
  • Cirrhosis of the liver
  • Malnutrition
  • Nephrotic syndrome
  • Albuminuria
  • Protein-losing enteropathy
  • Analbuminemia: a very rare condition due to defective mutation in the gene responsible for synthesis.

Albumin-Globulin Ratio

• In hypoalbuminemia, there will be a compensatory increase in globulins.
• Albumin-globulin ratio (A/G ratio) is altered or even reversed, leading to edema.

Hypergammaglobulinemias

• When albumin level is decreased, the body compensates by increasing the production of globulins.
• Chronic Infections: Gamma globulins are increased, but the increase is smooth and wide-based.
• Multiple Myeloma

Transport Proteins

• Albumin
• Pre-albumin or Transthyretin (Thyroxin binding pre-albumin (TBPA))
• Retinol binding protein (RBP)
• Thyroxine binding globulin (TBG)
• Transcortin (corticosteroid-binding globulin (CBG))
• Haptoglobin (Hp):
  • A hemoglobin-binding protein synthesized in the liver.
  • Prevents iron within hemoglobin from reacting with molecular oxygen to produce the free radical superoxide.

Acute Phase Proteins

• The level of certain proteins in the blood may increase 50 to 1000 folds in various inflammatory and neoplastic conditions.
• Examples:
  • C-Reactive Protein (CRP)
  • Ceruloplasmin (Cp)
    • Increased plasma Cp levels are seen in active hepatitis, biliary cirrhosis, hemochromatosis, obstructive biliary disease, pregnancy, estrogen therapy, inflammatory conditions, collagen disorders, and in malignancies.
    • Lowered level of ceruloplasmin is seen in Wilson's disease, malnutrition, nephrosis, and cirrhosis.

Wilson's Disease

• Level is reduced to less than 20 mg/dL in Wilson's hepatolenticular degeneration.
• It is an inherited autosomal recessive condition (incidence: 1 in 50,000).
• A genetic disorder that prevents the body from removing extra copper, causing copper to build up in the liver, brain, eyes, and other organs.
• Symptoms include:
  • Jaundice.
  • Kayser-Fleischer rings.

Symptoms of Wilson Disease

• Liver: Abdominal pain, dark urine or a light stool color, jaundice.
• Brain: Mood changes, anxiety and depression, disruptive thoughts and feelings.
• Eye: Rings around the edge of your corneas.
• Nervous System: Tremors, stiff muscles, problems with coordination.

Negative Acute Phase Proteins

• During an inflammatory response, some proteins are decreased in the blood.
• Examples: albumin, transthyretin (prealbumin), retinol binding protein, and transferrin.

Clinical Applications of Negative Acute Phase Proteins

• Albumin: Decreased in chronic inflammation, malnutrition, and liver disease.
• Transferrin: Decreased in inflammation and infection but increased in iron deficiency anemia.
• Transthyretin (Prealbumin): Used as a marker of nutritional status.

Clotting Factors

• Coagulation factors are present in circulation as inactive zymogen forms.
• They are converted to their active forms only when the clotting process is initiated.
• Several of these factors require calcium for their activation.
• Calcium ions are chelated by the gamma carboxyl group of glutamic acid residues of the factors: prothrombin, VII, IX, X, XI and XII.
• The gamma carboxylation of glutamic acid residues is dependent on vitamin K.

Prothrombin

• It is a single chain zymogen with a molecular weight of 72,000 D.
• The plasma concentration is 10–15 mg/dL.
• Prothrombin is converted to thrombin by Factor Xa, by the removal of N-terminal fragment.

Plasma Protein Listing of Clotting Factors

Thrombin

• It is a serine protease with molecular weight of 34,000 D.
• The Ca^{++} binding of prothrombin is essential for anchoring the prothrombin on the surface of platelets.
• When the terminal fragment is cleaved off, the calcium binding sites are removed, and so, thrombin is released from the platelet surface.

Fibrinogen

• The conversion of fibrinogen to fibrin occurs by cleaving of Arg-Gly peptide bonds of fibrinogen.
• Fibrinogen has a molecular weight of 340,000 D and is synthesized by the liver.
• Normal fibrinogen level in blood is 200–400 mg/dL.
• Fibrinogen is an acute phase protein.

Clinical Significance of Coagulation

• Thrombosis in the coronary artery is the major cause of myocardial infarction (heart attack).
• If TPA (Tissue plasminogen activator), urokinase, or streptokinase is injected intravenously in the early phase of thrombosis, the clot may be dissolved, and recovery of patient is possible.

Abnormalities in Coagulation: Hemophilia A

• This is an inherited X-linked recessive disease affecting males and transmitted by females.
• Male children of hemophilia patients are not affected, but female children will be carriers who transmit the disease to their male offspring.
• This is due to the deficiency of factor VIII (antihemophilic globulin) (AHG).
• It is the commonest of the inherited coagulation defects.

Answers: Serum Electrophoretic Patterns

• Normal pattern
• Multiple myeloma (M band) between beta and gamma region
• Chronic infection, broad based increase in gamma region; general increase in alpha1 and alpha2 bands
• Nephrotic syndrome; hypoalbuminemia; prominent alpha2 band
• Cirrhosis of liver; decreased albumin
• Plasma showing fibrinogen (normal condition). This may be mistaken for paraproteins