There are many widely used techniques for fractionating the serum proteins. “Salting out” by differential chemical solubility yields rough separation into albumin and globulin. Cohn devised a more complicated chemical fractionation method by which certain parts of the protein spectrum may be separated from one another in a large-scale industrial-type procedure. Albumin is most often assayed by a chemical method (biuret) that reacts with nitrogen atoms, or with a dye (such as bromcresol green or bromcresol purple) that preferentially binds to albumin. The ultracentrifuge has been used to study some of the subgroups of the globulins. This is possible because the sedimentation rate at high speeds depends on the molecular size and shape, the type of solvent used to suspend the protein, and the force of centrifugation. The velocity of any particular class of globulins under standard conditions depends primarily on molecular size and is known as the “Svedberg number”; the most common classes of globulins are designated as 7S, 19S, and 22S. Electrophoresis separates molecules by the electrical charge of certain structural atomic configurations and is able to subdivide the globulins, but only into groups rather than into individual proteins. Serum protein nomenclature derived from electrophoresis subgroups the serum globulins into alpha, beta, and gamma, corresponding to electrophoretic mobility. Using antibodies against antigens on the protein molecule, immunoassay (including radial immunodiffusion, Laurell “rocket” electroimmunodiffusion, immunonephelometry, immunofluorometry, and radioimmunoassay, among others) is another technique for measuring serum proteins that is assuming great importance. Immunoassay techniques quantitate individual proteins rather than protein groups and in general produce reliable results with excellent sensitivity and specificity. Immunoelectrophoresis or similar techniques such as immunofixation goes one step beyond immunoassay and separates some of the individual globulin molecules into structural components or into subclasses. Immunoelectrophoresis also can detect abnormal proteins that either differ structurally from normal proteins or are produced with different proportions of structural components.
Category: Serum Proteins
Serum Proteins
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Serum Proteins
Serum protein is composed of albumin and globulin, either nonbound or acting as carrier proteins. The word “globulin” is an old chemical fractionation term that refers to the non-albumin portion of serum protein; it was subsequently found that this portion includes a heterologous group of proteins such as glycoproteins, lipoproteins, and immunoglobulins. Most globulin molecules are considerably larger than albumin, although the total quantity of albumin is normally two to three times the level of globulin. Albumin seems most active in maintaining the serum oncotic pressure, where it normally has about 4 times as much importance as globulin, accounting for about 80% of the plasma oncotic pressure. Albumin also acts as a transport protein for some drugs and a few other substances. The globulins have more varied assignments than albumin and form the main transport system for various substances as well as constituting the antibody system, the clotting proteins, complement, and certain special-duty substances such as the “acute reaction” proteins. Most serum albumin is produced by the liver. Some of the globulins are produced by the liver, some by the reticuloendothelial system, and some by other tissues or by poorly understood mechanisms.
Plasma contains fibrinogen in addition to the ordinary serum proteins.