Plasma Proteins

Over 300 proteins have been detected in plasma .The concentration of many of these are affected by

pathological processes; they are therefore measured.They contain disulphide bonds.

Functions of plasma proteins include:• Transport• Maintaining plasma oncotic pressure• Buffering pH changes• Humoral immunity• Enzyme activity• Clotting• The acute inflammatory response.

Metabolism of Plasma Proteins:The concentration of plasma proteins (PP) is determined by 3

main factors:Rate of synthesisRate of catabolismThe volume of fluid in which proteins are distributed. A) Synthesis:Most plasma proteins are synthesized in the liver, although some

of them are produced in other sites e.g. immunoglobulinss by lymphocytes.

B) Distribution:

In health, the total concentration of proteins in plasma is around 70 g/L in normal adult males

Water passes more freely through capillary walls than proteins and therefore the concentration of proteins in the vascular space is affected by fluid distribution.

Application of a tourniquet for extended periods leads to fluid loss from the occluded veins; increasing apparently plasma protein concentrations.

C) Catabolism:PP are degraded throughout the body.Most proteins are degraded after being taken up by

cells within the body.

Assessment of PP:

2 types of techniques are used:Quantitative measurement of individual proteins or groups

of proteinsSemiquantitative assessment of the proportions of different

proteins present in plasma or serum.

A) Quantitative measurement:By chemical methods depending on the reaction of peptide

bonds with a chemical reagent. Individual proteins are usually determined by methods which use either chemical or immunological reactions.

B) Semiquantitative assessment:Proteins are separated from each other on the basis of their

electrical charge. In the technique of electrophoresis , serum is applied to a

support medium, usually cellulose acetate or a gel, and an electrical current applied. After a suitable period the medium is stained with a dye that visualizes the proteins. In normal subjects 5 discrete bands of proteins are seen and altered patterns occur in various diseases.

The most important abnormalities detected by electrophoresis are densely staining bands in the 2- region – these are paraproteins which result from monoclonal proliferations of B lymphocytes.

Specific Plasma Proteins PrealbuminPrealbumin migrates faster than albumin in the classic E/P.It can also be separated by high-resolution E/P or

immunoelectrophoresis techniques.Prealbumin is the transport protein for thyroid hormones and it

also binds with retinol-binding protein to form a complex that transports retinol (vitamin A).

Prealbumin is decreased in liver disease , nephrotic syndrome, acute phase inflammatory response and malnutrition.

It has a short half-life of approximately 2 days.

Albumin

Albumin is present in higher concentrations than other plasma proteins ( approximately 40 g/L in normal adults). Albumin has a molecular weight of approximately 66000 Daltons. Very small amounts cross the glomerular capillary wall.

Albumin is synthesized in the liver and has a half-life of approximately 20 days . Its synthesis is highly dependant on the supply of AA and therefore the production rate falls during periods of protein depletion.

The rate of catabolism of albumin increases as a result of injury, infection and surgery.

Functions:

The main function of albumin is maintenance of plasma oncotic pressure, transport and buffering pH changes.

Oncotic pressure: Albumin is responsible for approx. 80% of the plasma oncotic pressure. This is a major determinant of the distribution of fluids between intravascular and extravascular compartments. Hypoalbuminemia leads to edema.

Buffering. Transport: Many substances are transported in the blood bound to

albumin e.g. lipid-soluble substances, hormones e.g. thyroxine, calcium, drugs e.g. salicylates, nonesterified fatty acids (NEFA) and bilirubin.

  Reduced serum albumin levels are common in many conditions.

Hyperalbuminemia is rare and is usually caused by dehydration.

Causes of Hypoalbuminemia:

Artefacual : diluted specimens

Physiological:

Pregnancy Recumbency

Pathological:

Decreased production: Reduced availability of AA Manlnutrition Malabsorption

Defective synthesis: Analbuminemia Chronic liver diseases

Increased volume of distribution:

Increased plasma water: overhydrationRedistribution of Albumin:

Liver cirrhosis due to ascites

Increased Loss:

From the Kidney: Nephrotic syndromeFrom GIT: Protein losing enterpathiesIncresed Catabolismm:

SurgeryTraumaInfection

1-Antitrypsin:

It is synthesized by the liver and macrophagesIt inhibits proteasesProteases arise from several sources including

endogenous production, leukocytes and bacteria.Digestive enzymes such as trypsin and chymotrypsin are

released into the circulation in small amounts and other endogenous proteases include elastase and thrombin.

Infection leads to protease release from bacteria and from leucocytes

Deficiency of 1-Antitrypsin occurs as an inherited condition.

It may be detected by noting the absence of an 1 band on protein E/P or by measuring the protein specifically.

Genetic Polymorphisms of 1-Antitrypsin

Several variants of 1-Antitrypsin occur, the phenotypes being designed by the prefix Pi (Protease inhibitor) and over 30 alleles have been described, these being designed by a letter. The most common type is M.

Deficiency of 1-Antitrypsin is most commonly found in the PiZZ phenotype.

Synthesis of the defective 1-Antitrypsin occurs in the liver but there is a failure to secrete the protein , So it accumulates in hepatocytes and is deficient in plasma.

Clinical Consequences of 1-Antitrypsin deficiency:

Neonatal hepatitis with evidence of cholestasis, childhood cirrhosis and emphysema in young adults result from 1-Antitrypsin deficiency.

 

1-Fetoprotein: (AFT) Is synthesized in the developing embryo and fetus and then by the

parenchymal cells of the liver . AFT levels decreases gradually after birth, reaching adult levels by 8-12 months. However , AFT has no known function in normal adults.

The physiological function of AFT has not been completely identified but it has been proposed that the AFT protects the fetus from immunologic attack by the mother.

 Conditions associated with an elevated

maternal AFT levels include: Neural tube defects Spina bifida Anencephaly Low levels of maternal AFT indicates an increased risk for

Down syndrome. AFT is a tumor marker for hepatoma.

1-acid glycoprotein (Orosomucoid):

This protein is produced by the liver and is an acute phase reactant.

It is suggested that 1-Acid glycoprotein regulates immune responses.

Ceruloplasmin:Synthesized in the liverIts function is not clear.Contains over 90% of serum copper – the metal is tightly

bound and does not exchange readily.It is an oxidoreductase and this property is important in

acute phase response as it is able to inactivate reactive O2 species (ROS) that produce tissue damage. It is important for iron absorption from the intestine.

Plasma levels are usually low in Wilson’s disease in which copper is accumulated in the liver leading to cirrhosis , and in the basal ganglia of the brain leading to choreoathetosis.

Plasma level of ceruloplasmin are very sensitive to the effects of estrogens and rise during pregnancy and in response to oral estrogens

Haptoglobin:

It is synthesized by the liver

It binds free hemoglobin to form complexes that are metabolized in the RES, this has the effect of limiting Iron losses which may occur as Hb is small enough to be filtered by the glomerulus.

Its plasma level decreases during hemolysis and increases in acute inflammatory conditions (acute phase reactant).

2-Macroglobulin:

Synthesized in the liver & macrophages

It is a protease inhibitor for trypsin, thrombin and plasmin

It is a carrier for cytokines.

The plasma level of 2-macroglobulin increases in nephritic syndrome because its large size aids in its retention and also due to increased hepatic synthesis

Transferrin:• The major iron-tansporting protein in the plasma binding up to 2 atoms of iron per molecule of protein

• Normally , 30% saturated with iron.

• Its function is to transport iron from the intestine and between sites of synthesis and degradation of hemoglobin and other iron-containing proteins.

• Concentrations are reduced in malnutrition, liver disease, inflammatory conditions and malignancy.

• Iron deficiency results in increased hepatic synthesis.

• It is a negative acute phase protein

2 – Microglobulin: It is a component of human leukocyte Antigen (HLA).

Found on the surface of most nucleated cells and present in high concentration on lymphocytes.

Because of its small size, it is filtered by the renal glomeruli but most (>99%) is reabsorbed and catabolized in the proximal tubules.

Elevated serum levels are the result of impaired clearance by the kidney or overproduction of the protein that occurs in a number of inflammatory diseases such as rheumatoid arthritis and systemic lupus erythrematosus (SLE).

It may be used as a tumor marker for leukemia, lymphomas and multiple myeloma.

C-Reactive Protein: It is so named as it binds to the C polysaccharide of pneumococcal

cell walls.

It is synthesized by liver

It is important for phagocytosis

Very large increases in plasma C-reactive protein occur in many inflammatory conditions and is measured to detect and monitor diseases e.g. rheumatoid arthritis.

It is an acute-phase protein

Fibrinogen:

Is one of the largest proteins in the plasma.Synthesized in the liver.On plasma E/P, fibrinogen is seen as a distinct band

between the and globulins. Its function is to form a fibrin clot when activated by

thrombin therefore fibrinogen is virtually all removed in the clotting process and is not seen in serum.

It is one of the acute phase reactants. Its level rises with pregnancy and the use of oral

contraceptives.Decreased values generally reflects extensive coagulation

during which the fibrinogen is consumed.

Immunoglobulins: (Igs)Igs which are antibodies are a heterogeneous

group of plasma proteins produced by B lymphocytes.

Most are found in the region on E/P although some occur in the & 2 regions.

Five classes of Igs are recognized (M, A, G, E & D)

Hypergammaglobulinemia;Increases Ig levels may result from stimulation of many clones

of B cells (Polyclonal hypergammaglobulinemia) or monoclonal proliferation (Paraproteinemia).

 Polyclonal Hypergammaglobulinemia:Stimulation of many clones of B cells produce a wide range of

antibodies that appear as diffuse increase in -globulin on E/P.Acute and chronic infections , autoimmune diseases and

chronic liver diseases produce this type of response

Monoclonal Hypergammaglobulinemia:

Proliferation of a single B-cell clone produces a single Ig which appears as a discrete densely stained band (Paraprotein or M band) on E/P.

Paraproteins are characteristic of malignant B-cell proliferation.

Multiple myeloma is the commonest cause of paraproteinemia

Paraproteinemia is rare in chronic lymphayic leukemia and lymphomas.

Acute phase reactants: Stresses increases plasma protein as MI, inflammation , malignancy,

trauma or major surgery.

These proteins are termed acute phase reactants and their synthesis is a part of body’s response to injury.

They include;

1-Antitypsin (1-acid glycoprotein)

Haptoglobin

Ceruloplasmin

Fibrinogen

C-reactive protein,

These acute phase reactants are increased within 24 h of injury in response to humoral mediators (Cytokines – IL-1, IL-6, tumor necrosis factors and , the interferons, and platelet activating factors) which are produced by tissue macrophages, monocytes and endothelial cells.

Functions:

Binding to polysaccharides in bacterial wallsActivating complementStimulating phagocytosis

Humoral effects of IL-1 and IL-6 include increased production of ACTH and hence cortisol and inhibition of hepatic synthesis of proteins such as albumin, prealbumin and transferrin (negative acute phase proteins)

The protease inhibitors probably inactivate enzymes released from lysosomes and minimize damage that these may cause.