Plasma proteins, Study notes of Biochemistry

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Plasma proteins

• Include proteins of blood plasma and proteins of

interstitial fluid

• Almost all are glycoproteins
• Some groups of proteins are classified separately

(enzymes, proteohormones)

• Total protein more than 300 proteins Plasma proteins 1

§ Concentration 65 – 80 g ⁄ l;

• of this 35 – 50 g/l is albumin
• 20 – 35 g/l are serum globulins (transport protein, reactants of acute phase, globulins)
• Biosynthesis :
• liver (most), lymphocytes (immunoglobulins), enterocytes
• Degradation :
• hepatocytes, mononuclear phagocytic system (complexes of antigen-antibody, hemoglobin-haptoglobin) Plasma proteins 2

• Osmotic regulation:
  • Plasma proteins helps to maintain a normal blood volume and a normal water content in the interstitial fluid and the tissues.
  • Albumin content is most important in regulation of colloidal osmotic or oncotic pressure.
  • Decrease in albumin level results in loss of water from blood and its entry into interstitial fluids causing edema.
• Catalytic function (enzymes):
  • Lipases for removal of lipids from the blood Functions of plasma proteins 1

• Transport of substances :
  • albumin – fatty acids, bilirubin, calcium, drugs
  • transferrin – iron
  • cerulplasmin – copper
  • transcortin – cortisol, cortikosteron
  • lipoproteins – lipids
  • haptoglobin – free hemoglobin
  • thyroxin binding globulin – thyroxin
  • retinol binding protein - retinol Functions of plasma proteins 2

• Protective function:
  • Immunoglobulins combine with foreign antigens and remove them.
  • Complement system removes cellular antigens.
  • Enzyme inhibitors remove enzymes by forming complexes with them. For example a 1 - antitrypsin combines with elastase, trypsin and protects the hydrolytic damage of tissues such as lungs.
  • Some proteins increase during acute phase Functions of plasma proteins 4

Proteins move in an electric field according to their charge and size

Fractions Rel. amount (%) c (g/l) Albumins: albumin 52 – 58 34 – 50 a 1 - globulins: thyroxin-binding globulin, transcortin, a 1 - acid glycoprotein, a 1 - antitrypsin, a 1 - lipoprotein (HDL), a 1 - fetoprotein 2,4 – 4,4 2 - 4 a 2 - globulins: haptoglobin, macroglobulin, ceruloplasmin 6,1 – 10,1 5 – 9 b-globulins: transferrin, hemopexin, lipoprotein (LDL), fibrinogen, C-reactive protein, C3 and C components of the complement system 8,5 – 14,5 6 – 11 g-globulins: IgG, IgM, IgA, IgD, IgE 10 – 21 8 – 15

• Their levels change during acute inflammatory response
• Cause conditions where there is: - the destruction of cells - the reversible cell damage and subsequent repair - the metabolic activation of certain cells (immune cells)
• Concentration changes in: - infection - surgery - injury - cancer Acute phase reactants (APRs)

• Belongs to b 2 - globulin, the levels of which rise in response to inflammation
• Acute-phase reactant
• Plasma concentration levels of CRP rapidly increase within 2 hours of acute insult, reaching a peak at 48 hours (bacterial, viral, fungal infection, rheumatic diseases, malignity, tissue necrosis) C-reactive protein (CRP)

• Glycoprotein, belongs to b 2 - globulins (Mr 340 000)
• Concentration in plasma - 1.5 – 4.5 g/l
• Component of the coagulation cascade – fibrin

precursor

• Acute-phase reactant ⇒ ↑ acute inflammation Fibrinogen

§ In plasma: 300 mg/l § Functions: § carries 90% of copper in plasma ( copper – cofactor for a variety of enzymes ) 1 molecule binds 6 atoms of copper binds copper more tightly than albumin that carries other 10% of plasma copper Þ albumin may be more important in copper transport (donates copper to tissues more readily) Ceruloplasmin

• Transferrin is a β-globulin
• It binds free iron in serum
• Normally it is about one third saturated with iron
• Transferrin levels are decreased in :
  • liver disease (e.g. cirrhosis)
  • Chronic infections
  • Nephrosis
  • Congenitalatransferrinaemia
• Increased serum transferrin levels occur during increased transferrin synthesis caused as a result of iron deficiency anemia Transferrin