Plasma proteins

Plasma proteins are:

1. Serum albumin

2. Serum globulin

3. Fibrinogen.

Serum contains only albumin and globulin. Fibrinogen is absent in serum because, it is converted into fibrin during blood clotting. Because of this, the albumin and globulin are usually called serum

albumin and serum globulin.

NORMAL VALUES

Normal values of the plasma proteins are:

Total proteins : 7.3 g/dL (6.4 to 8.3 g/dL)

Serum albumin : 4.7 g/dL

Serum globulin : 2.3 g/dL

Fibrinogen : 0.3 g/dL

ALBUMIN/GLOBULIN RATIO

Ratio between plasma level of albumin and globulin is called albumin/globulin (A/G) ratio.

It is an important indicator of some diseases involving liver or kidney.

Normal A/G ratio is 2 : 1.

SEPARATION OF PLASMA PROTEINS

Plasma proteins are separated by the following methods.

1. PRECIPITATION METHOD

Proteins in the serum are separated into albumin and globulin. This is done by precipitating globulin with 22% sodium sulfate solution. Albumin remains in solution.

2. SALTING-OUT METHOD

Serum globulin is separated into two fractions called euglobulin and pseudoglobulin by salting out with different solutions. Euglobulin is salted out by full saturation with sodium chloride solution; half saturation with magnesium sulfate solution and onethird saturation with ammonium sulfate solution. It is insoluble in water. Pseudoglobulin is salted out by full saturation with magnesium sulfate and, half saturation with ammonium sulfate. It is soluble in water but it cannot be salted out by sodium chloride solution.

3. ELECTROPHORETIC METHOD

In this, the plasma proteins are separated depending on their differences in electrical charge and the rate of migration. It is done in a Tiselius apparatus by using paper or cellulose or starch block. By this method, the proteins are separated into albumin (55%), alpha globulin (13%), beta globulin (14%), gamma globulin (11%) and fibrinogen (7%).

4. COHN’S FRACTIONAL

PRECIPITATION METHOD

By this method, plasma proteins are separated into albumin and different fractions of globulin, depending

upon their solubility.

5. ULTRACENTRIFUGATION METHOD

In this method, albumin, globulin and fibrinogen are separated depending upon their density. This method is also useful in determining the molecular weight of these proteins.

6. GEL FILTRATION CHROMATOGRAPHY

Gel filtration chromatography is a column chromatographic method by which the proteins are separated on the basis of size. Protein molecules are separated by passing through a bed of porous beads. The diffusion of different proteins into the beads depends upon their size.

7. IMMUNOELECTROPHORETIC METHOD

By this method, the proteins are separated on the basis of electrophoretic patterns formed by precipitation at the site of antigenantibody reactions. This technique provides valuable quantitative measurement of different proteins.