Plasma Proteins Composition and Function

Plasma Proteins

• Plasma contains over 300 different proteins, with a concentration of approximately 6.0-8.0 g/L.
• Most plasma proteins are synthesized in the liver, while some are produced in other sites.

Types of Plasma Proteins

• There are three major fractions of plasma proteins: Albumin, Globulin, and Fibrinogen.
• These fractions can be further separated into:
  • Albumin (55.2%)
  • α1-Globulin (5.3%)
  • α2-Globulin (8.6%)
  • β-Globulin (13.4%)
  • γ-Globulin (11.0%)
  • Fibrinogen (6.5%)

Albumin

• Albumin is the most abundant plasma protein (~40 g/L) in normal adults.
• It is synthesized in the liver as prealbumin and secreted as albumin.
• About 40% of albumin is present in the plasma, and the other 60% is present in the extracellular space.

Functions of Albumin

• Maintains oncotic pressure, which pulls water into the circulatory system.
• Maintains fluid distribution in and outside cells and plasma volume.
• 80% of plasma oncotic pressure is maintained by albumin.
• It plays a predominant role in maintaining blood volume and body fluid distribution.
• Albumin also buffers pH and is a negative acute-phase reactant protein.
• It serves as a non-specific transport protein for various biomolecules.
• Albumin serves as a source of amino acids for tissue protein synthesis to a limited extent.

Abnormal Levels of Albumin and their Effects

Hypoalbuminemia

• Causes:
  • Inadequate source of amino acids (malnutrition and malabsorption)
  • Liver disease
  • Protein-losing enteropathy or gastrointestinal loss
  • Kidney loss to the urine in renal disease
  • Skin loss in the absence of the skin barrier
  • Hypothyroidism
  • Dilution by excess fluids
  • Acute disease states

Hyperalbuminemia

• Caused by dehydration
• No clinical conditions are known that cause the liver to produce large amounts of albumin.

Globulins

Types of Globulins

• α1-Globulins
• α2-Globulins
• β-Globulins
• γ-Globulins

α1-Globulins

• α1-Antitrypsin:
  • Synthesized by the liver and macrophages
  • Controls the proteolytic action of lysosomal enzymes from phagocytes
• α-Fetoprotein (AFP):
  • Synthesized in the developing embryo and fetus by the parenchymal cells of the liver
  • Function unknown, but may protect the fetus from immunologic attack by the mother
  • Elevated maternal AFP levels are associated with neural tube defects, anencephaly, spina bifida, and abdominal wall defects
  • Decreased maternal AFP levels are associated with an increased risk of Down's syndrome and trisomy 18

α2-Globulins

• Ceruloplasmin:
  • Synthesized by the liver
  • Carries >90% of serum copper
  • Binds copper very tightly, making it unavailable for exchange
• Haptoglobin (Hp):
  • Synthesized by the liver
  • Binds to free hemoglobin to form complexes that are metabolized in the RES
  • Prevents loss of free hemoglobin into the kidney, conserving valuable iron present in hemoglobin
  • Plasma level decreases during hemolysis and rises in inflammation