PLASMA PROTEINS

DR.GANGADHAR CHATTERJEE10/01/2013

Learning Objectives• To know different type of plasma

proteins present in human body.• Their distribution and functions• Methods of separation and estimation• To know each protein in greater depth• Clinical importance of the proteins• Diseases associated with these proteins

Plasma - blood treated with anticoagulants to prevent clot formation then centrifuged to remove the cells

Serum - blood that is allowed to form a fibrin clot then centrifuged to remove the cells and the clot

Plasma

“Buffy coat”

Red blood cells

PlasmaBuffy coatRed blood cells

Figure 19.1b

The Composition of Whole Blood

Plasma proteins• include proteins of blood plasma and

proteins of interstitial fluid

• almost all are glycoproteins

• some groups of proteins are classified separatelly (enzymes, proteohormones)

• “total protein” ~ more than 300 proteins

Individual proteins of blood plasma

The figure is from http://www.beckmancoulter.com/products/instrument/protein/proteomelab_igy_dcr.asp (Feb 2007)

Classification of plasma proteins

• by electrophoretic mobility

prealbumins albumin alpha, beta and

gamma-globulins fibrinogen

The figure is from textbook: Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th ed. Wiley‑Liss, Inc., New York, 1997. ISBN 0‑471‑15451‑2

The figure is from textbook: Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th ed. Wiley‑Liss, Inc., New York, 1997. ISBN 0‑471‑15451‑2

Principal proteins of each fraction

1-antitrypsin

orosomucoid

2-macroglobulin

haptoglobin

transferrin

C3-complement

immunoglobulins: IgG, IgA, IgM

• by specific function

transport proteins

proteins of immune system

system of proteases and antiproteases

proteins of hemocoagulation system

signal proteins

enzymes

cellular proteins

• by clinical use

cardiomarkers

tumormarkers

acute phase reactants

cellular enzymes

hormones

cytokines

protein Mr(x 103)

albumin 66transferrin 80haptoglobin 1-1 85IgG 144IgA 160haptoglobin 2-2 1602-macroglobulin 720IgM 971

(accepted from book: Clinical Laboratory Diagnostics / Lothar Thomas)

The plasma proteins

are found into three

major groups :

• fibrinogen• albumin• globulins

Relative dimension and approximate molecular masses of protein molecules in the blood

Some Functions of Plasma Proteins

General properties of plasma proteins

Most are synthesized in the liverexception: -globulins – synthesized in plasma cells

Synthesized as pre-proteins on membrane-bound polyribosomes; then they are subjected to posttranslational modifications in ER and Golgi apparatus

Almost all of them are glycoproteinsException: albumin

They have characteristic half-life in the circulation (albumin – 20 days)

Many of them exhibit polymorphism (immunoglobulins,

transferrin…)

Proteins of interstitial fluid• subcutaneous: albumin

• lymph: less proteins than in plasma

• liquor: 200x less than in plasma

• pathological fluids: transudate < 30 g/l exudate > 30 g/l

Factors influencing concentration of proteins

total protein: 64 – 83 g/l

• rate of synthesis and degradation

• distribution in body fluids

• loss into the third space

• elimination from the body

• hydration of the body

other important factors:• elevation of concentration before taking

blood sample

body position ( in supine position)

tightening of arm

• storage of biological specimens

Physiological variability• increased concentrations

plasma > serum (fibrinogen) stand-up position (by 10-15 %) increased muscle activity (by 12 %) dehydration

• decreased concentrations children, pregnant women after starvation (albumin, transferrin, C3)

Regulation of synthesis INCREASE

inflammation hyperthyroidism hypercortisolism Growth hormone iron deficiency protein loss clonal production

of Ig

DECREASE

liver damage with parenchymal tissue

nutritional deficit hypothyroidism diabetes mellitus alcoholism

Reversible Equilibrium Between the Plasma

Proteins and the Tissue Proteins.

Methods of separation

• Precipitation by SALTING OUT:• Different concentration of salt solutions are used to

precipitate different concentration of protein.• Salts most commonly used - Ammonium sulphate solution -Mixture of Sodium Sulphate and Sodium Sulphite

On 1/5th saturation with ammonium sulphate FIBRINOGEN is best precipitated.

On 1/3rd saturation globulin starts precipitating.It is called Euglobulin.

On ½ saturation total globulin is precipitated out. It is called Pseudoglobulin.

Albumin is precipitated out by full saturation.

• Fractionation of plasma proteins by ETHANOL [Cohn’s Fractionation]

Varying concentration of ethanol at low temperature Cohn’s Fractions

Fraction I rich in FibrinogenFraction II γ-globulinFraction III α and β globulins

Including isoaglutinnin & prothrombin

Fraction IV α &β globulinsFraction V albumin

Advantage of this fractionation Ethanol that is used for separation can be readily

removed by evaporation. Being a mild procedure it does not cause

denaturation of protein.

Clinically Cohn’s method is used for obtaining purified proteins on large scale for therapeutic purpose.

ELECTROPHORESIS:Initially paper electrophoresis was used.Nowadays agar gel electrophoresis, cellulose acetate membrane electrophoresis, immunoelectrophoresis is used.

• Proteomic analysis:• Nephelometry• Immunoturbimetry• Gel based separation technique e.g. 2-D PAGE• Mass spectrometry• MALDI-TOF

Many Plasma Proteins Exhibit PolymorphismHuman plasma proteins that exhibit polymorphism include

1- antitrypsin,haptoglobin,transferrin,ceruloplasmin, andimmunoglobulins

The polymorphic forms of these proteins can be distinguished by different procedures (eg, various types of electrophoresis or isoelectric focusing), in which each form may show a characteristic migration.

Analysis of these human polymorphisms have proved to be of genetic, anthropologic, and clinical interest.

ALBUMIN Albumin (69 kDa) is the major protein of human

plasma. Normal level is 3.5 - 5.5g/dL Makes up approximately 60% of the total plasma

protein. About 40% of albumin is present in the plasma,

and the other 60% is present in the extracellular space.

The liver produces about 12g of albumin per day. Gene for albumin is situated at chromosome no. 4

Albumin is synthesized by hepatocytes as preproalbumin. Its signal peptide is removed, converting it to proalbumin. In turn, proalbumin, while inside transport vesicles, is converted to albumin by action of furin.

Furin is a enzyme that cleaves a hexapeptide from proalbumin immediately C-terminal to a dibasic amino acid site (Arg). The resulting mature albumin is secreted into the plasma.

The synthesis of albumin decreases relatively early in conditions of protein malnutrition, such as kwashiorkor.

In liver diseases the synthesis of albumin is depressed (decrease in albumin/globulin ratio).

Signal Peptide Furin

Preproalbumin Signal Peptide Hexapeptide + + Proalbumin ALBUMIN

Structure and Functions of Albumin Mature human albumin consists of one polypeptide chain

(simple protein) of 585 amino acids and contains 17 disulfide bonds.

Albumin has an ellipsoidal shape, which means that it does not increase the viscocity of the plasma.

Because of its relatively low molecular mass (about 69 kDa) and high concentration albumin is thought to be responsible for 70-80% of the osmotic pressure of human plasma.

By the use of proteases, albumin can be subdivided into three domains, each have different functions.

.

maintenance of colloidal oncotic pressureTransport of long chain fatty acids & sterols Transport of bilirubin Binding and solubilization of drugs

Functions of Albumin

HypoalbuminemiaLow blood albumin levels (hypoalbuminemia) can be caused by:•Liver disease; cirrhosis of the liver is most common•Excess excretion by the kidneys (as in nephrotic syndrome)•Excess loss in bowel (protein-losing enteropathy)•Burns (plasma loss in the absence of skin barrier)•Redistribution (hemodilution [as in pregnancy])•Acute disease states (referred to as a negative acute-phase protein)

Hyperalbuminemia-Due to severe or chronic dehydration and high protein diets. - Chronic dehydration needs to be treated with zinc as well as with water. - Zinc reduces cell swelling caused by increased intake of water (hypotonicity) and also increases retention of salt.-In the dehydrated state, the body has too high in osmolarity and, it discards zinc to prevent this.

GLOBULINS

• α-globulins:• Further classified into α1 and α2 globulin

according to electrophoretic mobility. α1 acid glycoprotein: Also called orosomucoid Normal level 0.6 – 1.4 gm/lit Carbohydrate content is about 41% Considered as a reliable indicator of acute

inflammation binds to progesterone hormone and transport it.

• α- fetoprotein:• Present in high conc. in fetus particularly in mid

pregnancy. • Normal adult level < 1µg /100ml.• Potential tumour marker of hepatocellular

carcinoma.• Forms the basis of TRIPLE test of screening for

DOWNS syndrome.

1-Antitrypsin (about 52 kDa) is a single chain protein of 394 amino acids, contains three olygosaccharide chains.

It is synthesized by hepatocytes and macrophages and is the principal serine-protease inhibitor (Serpin or Pi) of human plasma.

Inhibits trypsin and elastase.

α1-ANTITRYPSIN (1-ANTIPROTEINASE)

Deficiency of Antitrypsin-IAt least 75 polymorphic forms occur, many of

which can be seperated by electrophoresis.

The major genotype is MM and its phenotypic product is PiM.

A deficiency of this protein has a emphysema.This occurs in subjects with the ZZ genotype who

syntyhesize PiZ and also in PiSZ heterozygotes.

When the amount of 1-antitrypsin is deficient and polymorphonuclear white blood cells increases in the lung (eg, during pneumonia), the affected individual lacks a countercheck to a proteolytic damage of lung by proteases such as elastase.

Methionine (358 residue) of 1-antitrypsin is involved in its binding to proteases.

Smoking oxidizes this methionine to methionine sulfoxide and this results in proteolytic destruction of lung tissue (eg, PiZZ; already have a low levels of 1-antitrypsin).

Deficiency of 1-antitrypsin is also implicated in one type of liver of liver disease (in ZZ type).

Conformational diseases (Loop-sheet polymerization): Most appear to be due to the formation of β-sheets by conformationally unstable proteins, which in turn leads to formation of aggregates.

At present, severe 1-antitrypsin deficiency liver disease can be successfully treated by liver transplantation.

Deficiency of Antitrypsin-II

CERULOPLASMIN [α-2 globulin] Conc. in plasma: 300 mgl It is a glycoprotein with enzyme activity ( copper oxidase,

histaminase, ferrous oxidase )

Functions: carries 90% of copper in plasma (copper – cofactor for a

variety of enzymes);1 molecule binds 8 atoms of copper; [½ as cuprous and ½ as cupric ]binds copper more tightly than albumin that carries other 10% of copper albumin may be more important in copper transport (donates copper to tissues more readily)

Causes of ceruloplasmin decrease: Liver diseases, in particular Wilson´s disease:

genetic disease in which copper fails to be excreted into the bile and accumulates in liver, brain, kidney, and red blood cells

cause: mutations in the gene encoding for copper-binding ATPase consequences:

• accumulation of copper in liver, brain, kidneys… liver disease, neurologic symptoms

• ↓ coupling of copper to apoceruloplasmin low plasma levels of ceruloplasmin

Causes of ceruloplasmin increase:

Inflammatory states Carcinomas, leukaemia Rheumatoid arthritis

Haptoglobin (Hp) is a plasma glycoprotein that binds extracorpuscular hemoglobin (Hb) in a tight noncovalent complex (Hb-Hp). Haptoglobin binds to extracorpuscular hemoglobin, preventing free hemoglobin from entering the kidney.

Hb-Hp complex is too large to pass through the glomerulus. The function of Hp thus appears to be to prevent loss of free Hb into the kidney.

HAPTOGLOBIN (Hp) [α-2 globulin]

Different fates of free hemoglobin and of the Hb-Hp complex

Polymorphic forms of Haptoglobin

Human haptoglobulin exists in three polymorphic forms, known as Hp 1-1, Hp 2-1, and Hp 2-2.

Hp1-1,is the simplest polymorphic form in human. Hp1-1migrates in starch gel electrophoresis as a single band, whereas Hp2-1and Hp2-2 exhibit much more complex patterns. It has been suggested that the haptoglobuin polymorphism may be associated with the prevalence of many inflammatory diseases.

Free Hb passes through the glomerulus of the kidney, enters the tubules, and tends to precipitate therein.

However the Hb-Hp complex is too large to pass through the glomerulus.

The function of Hp thus appears to be to prevent loss of free Hb into the kidney.

This conserves the valuable iron present in hemoglobin, which would otherwise be lost to the body.

Functions of Haptoglobin

Low levels of haptoglobin are found in patients with hemolytic anemias. It evaluates the degree of INTRAVASCULAR HEMOLYSIS.

Half-life of haptoglobin is approximately 5 days.The half of the Hb-Hp complex is cleared in about 90 minutes,

the complex being rapidly removed from plasma by hepatocytes.

Haptoglobin-related protein is another protein found in the plasma. Its level is elevated in some patients with cancers.

Certain other plasma proteins bind heme but not hemoglobin. Hemopexin is a β1-globulin that binds free heme.

Metheme (ferric heme) forms methemalbumin; which transfers the metheme to hemopexin.

Functions of Haptoglobin

β-GLOBULIN• β-LIPOPROTEIN: Already discussed in lipid

metabolism• Haemopexin: level is very low at birth but reaches adult value by first year of life. normal value: 0.5 – 1.0 g/lit bind and remove circulating haem which is formed in the body from breakdown of Hb, myoglobin or catalase. decrease in hemolytic disorders.

Transferrin (Tf) is a β1-globulin with a molecular mass of approximately 76 kDa.

It is a glycoprotein and is synthesized in the liver.

About 20 polymorphic forms of transferrin have been found.

Transferrin plays a central role in the body’s metabolism of iron because it transports iron (2 mol of Fe3+ per mole of Tf) in the circulation to sites where iron is required, eg, from the gut to the bone marrow and other organs.

Approximately 200 billion red blood cells (about 20 mL) are catabolized per day, releasing about 25 mg of iron into the body.

TRANSFERRIN

Transferrin ReceptorsThere are receptors (TfR1 and TfR2) on the surfaces

of many cells for transferrin. It binds to these receptors and is internalized by receptor-mediated endocytosis.

The acid pH inside the lysosome causes the iron to dissociate from the protein. The dissociated iron leaves the endosome via DMT1 to enter the cytoplasm.

Abnormalities of the glycosylation of transferrin occur in the congenital disorders of glycosylation and in chronic alcohol abuse.

Receptor-mediated transferrin

endocytosis

Ferro-transferrin binds to the receptors on the cell surface → the complex is internalized into an endosome

In endosomes, iron dissociates from trans-ferrin (enabled by low pH & Fe3+ Fe2+ reduction) and enters cytoplasm

Iron is delivered to intracellular sites or bound to ferritin (Fe2+ Fe3+ oxidation and Fe3+ storage)

Apotransferrin, associated with the receptor, returns to the membrane, dissociates from the receptor and re-enters plasma.

Causes of transferrin deficiency:

Burns Infections Malignancies Liver and kidney diseases

Cause of relative transferrin excess:

Iron-deficiency anaemia

C-reactive protein (an acute-phase protein)- It is a protein found in the blood, the levels of which rise

in response to inflammation.- Its role is to bind to phosphocholine on the surface of

dead or dying cells and some types of bacteria to activate the complement system.

- CRP is synthesized by the liver in response to factors released by adipocytes and not related to C-peptide or protein C.

• -It is used mainly as a marker of inflammation and in cancer, which is not clear until now.

- Measuring CRP values by ELISA, rapid immunodiffusion and visual agglutination is useful in determining disease progress or the effectiveness of treatments.

β-2 MICROGLOBULIN• Present In urine to the extent only 0.01mg/ 100ml• Close resemblance with immunoglobulins.• Increased in renal disease and it is a reflection of

impairment of function of glomerular membrane or renal tubules.

• Now receiving much attention as a tumour marker.

• Useful to check renal damage by gentamycin therapy.

Acute phase reactants (APRs)

Their levels change during acute inflammatory response

APRs concentration changes in: infection surgery injury cancer

Acute phase reactant response

Types of APRs:

Negativealbumintransferrin

Positive1-antitrypsinC-reactive protein (CRP): ~1000-fold increase!fibrinogenhaptoglobin (HP)C3, C4

Fibrinogen (0.2-0.45 g/dl) representing 4% of plasma proteins-It is a soluble plasma glycoprotein, that is converted by thrombin into fibrin during blood coagulation. -This is achieved through processes in the coagulation cascade that activate the zymogen prothrombin to the serine protease thrombin, which is responsible for converting fibrinogen into fibrin, which is then cross linked by factor XIII to form a clot. -FXIIIa stabilizes fibrin further by incorporation of the fibrinolysis inhibitors alpha-2-antiplasmin and binding to several adhesive proteins of various cell

2-Macroglobulin is a large plasma glycoprotein (720 kDa) made up of 4 identical subunits of 180kDa.

Approximately 10% of the zinc in plasma is transported by 2-macroglobulin , the remainder being transported by albumin.

α-2 MACROGLOBULIN

The protein is synthesized by a variety of cell types, including monocytes, hepatocytes and astrocytes.

It is the major member of a group of plasma proteins that include complement proteins C3 and C4.

These proteins contain a unique internal cyclic thiol ester bond (formed between cysteine and glutamine residue) and for this reason have been designated as the thiol ester plasma protein family.

2-macroglobulin binds many proteinases and is thus an important panproteinase inhibitor.

The 2-macroglobulin – proteinase complexes are rapidly cleared from the plasma by a receptor located on many cell types.

2-macroglobulinbinds many cytokines (platelet derived growth factor, transforming growth factor β, etc..) and appears to be involved in targeting them toward particular tissues or cells.

Once taken up by cells, the cytokines can dissociate from 2-macroglobulin and subsequently exert a variety of effects on cell growth and function.

GENETIC DEFICIENCIES OF PLASMA PROTEINS

ANALBUMINAEMIA: Inherited disorder; very low or complete absence

of Albumin. Defect is in the albumin synthesis a/w raised lipid and lipoprotein level. All globulin fraction increased.BISALBUNEMIA: Two albumin peaks No clinical importance

BRUTON’S AGAMMAGLOBULINEMIA: X-linked recessive disorder Differentiation of B-lymphocytes to plasma cells is

defective leading to lack of plasma cells in circulating blood.

Absence of γ-globulin or very low level. Lack of humoral immunity and susceptible to

bacterial infections.AFIBRINOGENEMIA: Genetic defect in fibrinogen formation. Absence of fibrinogen or very low level. Blood clotting mechanism is hampered. May be uncontrollable hemorrhage.

BIBILOGRAPHY• TIETZ’S CLINICAL CHEMISTRY• THOMAS DEVLIN BIOCHEMISTRY WITH

CLINICAL CORRELATION• HARRISON’S PRINCIPLE OF INTERNAL

MEDICINE• ROBBIN’S PATHOLOGIC BASIS OF

DISEASES• GUYTON’S MEDICAL PHYSIOLOGY• BAYNES MEDICAL BIOCHEMISTRY

THANK YOU