Albumin and Plasma Proteins

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Plasma Proteins Overview - The Blood's Workhorses

Total plasma protein: 6.0-8.3 g/dL. Primarily synthesized in the liver (except γ-globulins from plasma cells).

  • Core Functions:

    • Oncotic pressure (key: albumin)
    • Transport (e.g., hormones, drugs, lipids)
    • Buffering (acid-base balance)
    • Immunity (γ-globulins/antibodies)
    • Coagulation (e.g., fibrinogen)
  • Major Classes & Primary Roles:

    • Albumin: Highest concentration; maintains oncotic pressure, transport.
    • Globulins:
      • α1, α2: Transport, protease inhibitors.
      • β: Transport (e.g., iron via transferrin, lipids via LDL).
      • γ (Immunoglobulins): Antibodies for immunity.
    • Fibrinogen: Essential for blood clotting.
  • Analysis - SPEP: Serum Protein Electrophoresis separates into 5 bands: Albumin, α1, α2, β, and γ globulins.

⭐ Most plasma proteins are glycoproteins, except for albumin.

Albumin Focus - Osmotic & Transport Titan

  • Most abundant plasma protein (~60% of total; 3.5-5.0 g/dL).
  • Structure: Single polypeptide chain; no carbohydrate (CHO).
  • Functions:
    • Major contributor to plasma oncotic pressure (~80%).
    • Versatile transport: Bilirubin, fatty acids, Ca²⁺, hormones, drugs (e.g., warfarin, salicylates).
    • Buffering (minor physiological role).
  • Synthesis: Exclusively in the liver.
  • Acute Phase Reactant: Negative (levels ↓ during inflammation).
  • Hypoalbuminemia (<3.5 g/dL):
    • Causes: 📌 NALS (Nephrotic syndrome, Anorexia/Malnutrition, Liver cirrhosis, Severe burns/Skin loss).
    • Consequences: Generalized edema (peripheral, pulmonary, ascites).

    ⭐ Edema typically occurs when serum albumin levels fall below 2.0-2.5 g/dL.

  • Hyperalbuminemia (>5.0 g/dL):
    • Causes: Primarily severe dehydration.
  • Analbuminemia: Rare congenital disorder (virtual absence of albumin).

Plasma Proteins Composition Diagram

Globulins Deep Dive - Defense & Delivery Crew

  • Heterogeneous group; separated by electrophoresis (e.g., SPEP).
TypeKey Proteins & Core FunctionClinical Correlations
α1-Globulins- α1-Antitrypsin (AAT): Protease inhibitor
- α-Fetoprotein (AFP): Fetal marker
- AAT def: Emphysema, liver dz
- AFP: ↑ HCC, neural tube defects (NTDs)
α2-Globulins- Ceruloplasmin: Cu transport, ferroxidase
- Haptoglobin: Binds free Hb
- α2-Macroglobulin: Protease inhib.
- Ceruloplasmin: ↓ Wilson's dz
- Haptoglobin: ↓ Hemolysis
- α2-Macro: ↑ Nephrotic synd.
β-Globulins- Transferrin: Fe transport
- β2-Microglobulin: MHC I component
- CRP: Major APR
- Transferrin: ↑ Fe deficiency anemia
- β2-Micro: ↑ Renal failure, malignancies
- CRP: ↑ Inflammation
γ-Globulins- Immunoglobulins (Igs e.g., IgG, IgM): Antibodies from plasma cells- Altered in immune disorders, infections, paraproteinemias (e.g., Myeloma)

⭐ A decreased or absent α1-globulin band on SPEP is highly suggestive of α1-antitrypsin deficiency.

Fibrinogen & Others - Clotting & Crisis Responders

  • Fibrinogen (Factor I): Synthesized in the liver. Crucial for blood coagulation; converted to fibrin by thrombin.

    • Normal range: 200-400 mg/dL.
    • Conditions: Afibrinogenemia (absence), dysfibrinogenemia (abnormal function).
  • Acute Phase Reactants (APRs): Proteins whose plasma levels change significantly with inflammation/tissue injury (stimuli: IL-1, IL-6, TNF-α).

    • Positive APRs (↑): C-Reactive Protein (CRP), Fibrinogen, Haptoglobin, Ceruloplasmin, Alpha-1 Antitrypsin (A1AT), Ferritin, Serum Amyloid A (SAA).
    • Negative APRs (↓): Albumin, Transferrin, Prealbumin (Transthyretin).
  • Plasma Protein Electrophoresis (SPEP) Patterns:

    • Nephrotic Syndrome: ↓ Albumin, ↑ α2-macroglobulin, ↓ γ-globulins.
    • Liver Cirrhosis: Polyclonal gammopathy (broad ↑ γ-globulins), β-γ bridging.
    • Multiple Myeloma: Monoclonal gammopathy (M-spike). SPEP patterns in nephrotic syndrome, cirrhosis, myeloma

⭐ In multiple myeloma, the M-spike is most commonly due to monoclonal IgG, followed by IgA.

High‑Yield Points - ⚡ Biggest Takeaways

  • Albumin: Most abundant plasma protein, synthesized in the liver; maintains oncotic pressure and acts as a transporter.
  • Hypoalbuminemia: Key cause of generalized edema.
  • α1-Antitrypsin deficiency: Associated with early-onset emphysema and liver cirrhosis.
  • Ceruloplasmin: Primary copper-carrying protein; decreased in Wilson's disease.
  • Haptoglobin: Binds free hemoglobin; levels decrease significantly in intravascular hemolysis.
  • Transferrin: Major iron transport protein; levels increase in iron-deficiency anemia.
  • Acute phase reactants (e.g., CRP, fibrinogen) are altered during inflammation; albumin is a negative acute phase reactant (levels ↓).

Practice Questions: Albumin and Plasma Proteins

Test your understanding with these related questions

What is the Child-Pugh class for a patient who has a serum bilirubin of 2.5 mg/dL, serum albumin of 3 g/dL, INR of 2, mild ascites, but no encephalopathy?

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Flashcards: Albumin and Plasma Proteins

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There are four superfamilies of proteins namely _____, dynein, myosin, dynamins referred to as molecular motors.

TAP TO REVEAL ANSWER

There are four superfamilies of proteins namely _____, dynein, myosin, dynamins referred to as molecular motors.

kinesin

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