Untitled Quiz

Plasma Proteins, Functions, and Separation Techniques

• Plasma contains over 300 different proteins.
• Many pathological conditions affect plasma protein levels.
• Most plasma proteins are synthesized in the liver.
• Some proteins are synthesized in other locations.
• A normal adult contains approximately 70 g/L of plasma proteins.

Outline

• Functions and characteristics of plasma proteins
• Measurement of plasma proteins and disease diagnosis
• Electrophoretic patterns of plasma proteins
• Acute phase proteins

Plasma Proteins

• Transport proteins (albumin, prealbumin, globulins) maintain plasma oncotic pressure and transport hormones, calcium, free fatty acids, and drugs.
• Defense proteins (immunoglobulins, complement) are involved in immunity.
• Clotting and fibrinolysis proteins (thrombin, plasmin) are involved in blood clotting and its breakdown, respectively.

Measurement of Plasma Proteins

• A) Quantitative Measurement: Chemical or immunological reactions are used to measure specific proteins.
• B) Semiquantitative Measurement: Electrophoresis is used to separate proteins based on their electrical charge, creating five distinct bands that change in disease states.

Normal Pattern of Plasma Protein Electrophoresis

• The bands have specific protein components.
• Examples include albumin, α₁-antitrypsin, TBG, transcotrin, haptoglobin, α₂ macroglobulin, ceruloplasmin, transferrin, β-lipoprotein, antibodies.

Types of Plasma Proteins

• Prealbumin (Transthyretin): Transports thyroid hormones and retinol; migrates faster than albumin in electrophoresis; low levels indicate liver disease, nephrotic syndrome, and inflammation/malnutrition.
• Albumin: Most abundant plasma protein (~40 g/L); synthesized in the liver; half-life is 20 days; decreases rapidly in injuries, infections, and surgeries; maintains oncotic pressure (80%).
• a₁-Globulins: Includes α₁-antitrypsin, α-fetoprotein.
• a₂-Globulins: Includes ceruloplasmin and haptoglobin
• ß-Globulins: Includes CRP, transferrin, β₂-microglobulin.
• γ-Globulins: Include antibodies

Prealbumin (Transthyretin)

• It is a transport protein for thyroid hormones and retinol
• It migrates faster than albumin in electrophoresis
• It's separated by immunoelectrophoresis
• Low levels are found in liver disease, nephrotic syndrome, acute phase inflammatory response, and malnutrition.
• It has a short half-life of 2 days.

Albumin

• It is the most abundant plasma protein (~40 g/L) in normal adults.
• It is synthesized in the liver as preproalbumin and secreted as albumin.
• It has a half-life in plasma of 20 days.
• It decreases rapidly in injury, infection, and surgery.
• It plays a critical role in maintaining oncotic pressure, influencing fluid distribution in and around cells.

Functions

• Maintains oncotic pressure; this pressure exerted by plasma proteins pulls water into the circulatory system, maintaining fluid distribution, and plasma volume.
• Albumin is responsible for 80% of plasma oncotic pressure.
• Acts as a non-specific carrier for hormones, calcium, free fatty acids, drugs, etc.
• Tissue cells can take up albumin by pinocytosis and use it to synthesize amino acids.
• Useful in treating liver diseases, hemorrhage, shock, and burns.

Hypoalbuminemia

• Causes: Decreased albumin synthesis (liver cirrhosis, malnutrition) and increased albumin loss (excessive catabolism, infections, kidney diseases (nephrotic syndrome), and bowel dysfunction, severe burns)
• Effects: Edema (low oncotic pressure), reduced transport of drugs and other substances, decreased protein-bound calcium (total calcium drops, but ionized calcium might remain normal).

Hyperalbuminemia

• No known clinical conditions result in increased albumin production; dehydration is the only known cause.

a₁-Antitrypsin

• Synthesized by the liver and macrophages.
• It's an acute-phase protein that inhibits proteases (digestive enzymes like trypsin, chymotrypsin, and other proteases like elastase and thrombin) released by leukocytes and bacteria during infection.

Types of α₁-Antitrypsin

• Over 30 types are known; the most common is M type.
• Genetic deficiency of α₁-antitrypsin; synthesis in the liver, but secretion fails, leading to accumulation in hepatocytes and reduced plasma levels.

Clinical Consequences of a₁-Antitrypsin Deficiency

• Neonatal jaundice and cholestasis
• Childhood liver cirrhosis.
• Pulmonary emphysema in young adults.

Laboratory Diagnosis of a₁-Antitrypsin Deficiency

• Lack of α₁-globulin band in protein electrophoresis.
• Quantitative measurement of α₁-antitrypsin using methods like radial immunodiffusion, isoelectric focusing, or nephelometry.

α-Fetoprotein (AFP)

• Synthesized by the developing embryo and fetus (liver parenchymal cells).
• AFP levels decrease during intrauterine life.
• Function unknown but thought to protect the fetus from immune attack by the mother.
• No known physiological function in adults

α-Fetoprotein (AFP) - Clinical Significance

• Elevated maternal AFP levels are associated with neural tube defects and anencephaly
• Decreased maternal AFP levels may increase the risk of Down's syndrome.
• AFP is a tumor marker for hepatocellular carcinoma (hepatomas) and testicular cancer.

Ceruloplasmin

• Synthesized by the liver.
• Contains over 90% of serum copper.
• Oxidoreductase that inactivates Reactive Oxygen Species (ROS) causing tissue damage (important in the acute phase response).
• Important for iron absorption from the intestine.

Ceruloplasmin deficiency (Wilson's disease)

• Low plasma ceruloplasmin levels cause copper accumulation in the liver and brain.

Haptoglobin

• Synthesized by the liver.
• Binds free hemoglobin to prevent kidney loss; complexes are metabolized by the reticuloendothelial system.
• Plasma levels decrease during hemolysis.

Transferrin

• A major iron-transport protein in plasma; usually 30% saturated with iron.
• Plasma level drops in malnutrition, liver disease, inflammation, and malignancy.
• Iron deficiency increases hepatic transferrin synthesis.
• A negative acute phase protein.

β₂-Microglobulin

• A component of the human leukocyte antigen (HLA).
• Present on the surface of lymphocytes and most nucleated cells.
• Filtered by the renal glomeruli, but most (over 99%) is reabsorbed.
• Elevated levels are found in impaired kidney function, overproduction in diseases, and may be a tumor marker (leukemia, lymphomas, multiple myeloma).

C-Reactive Protein (CRP)

• An acute-phase protein synthesized by the liver.
• Important for phagocytosis.
• High plasma levels indicate inflammatory conditions (e.g., rheumatoid arthritis).
• Marker for ischemic heart disease.

Hypergammaglobulinemia

• May result from B-cell stimulation (polyclonal) or monoclonal proliferation (paraproteinemia).
• Polyclonal: Stimulation of many B-cell clones results in a wide range of antibodies, creating a broad y-globulin band in electrophoresis; associated with acute and chronic infections, autoimmune diseases, and chronic liver diseases.
• Monoclonal: Proliferation of a single B-cell clone results in a single type of immunoglobulin (Ig) producing a dense band (paraprotein or M band) in electrophoresis; characteristic of malignant B-cell proliferations (e.g., multiple myeloma).

Positive Acute Phase Proteins

• Plasma protein levels increase during infection, inflammation, malignancy, trauma, and surgery.
• These proteins are called acute-phase reactants.
• Their synthesis is due to the body's response to injury.
• Examples: α₁-antitrypsin, haptoglobin, ceruloplasmin, fibrinogen, CRP.

Negative Acute Phase Proteins

• These protein levels decrease during inflammation.
• Examples: Albumin, prealbumin, transferrin.
• This decrease is due to the body utilizing amino acids for the synthesis of positive acute-phase proteins.

Functions of Positive Acute-Phase Proteins

• Binding to polysaccharides in bacterial cell walls.
• Activate the complement system.
• Stimulate phagocytosis by immune cells.