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Plasma Proteins

Although the proteins in all fluid compartments play a major physiologic function, the plasma proteins are the most frequently analyzed. More than 500 plasma proteins have been identified.

Prealbumin (Transthyretin) Prealbumin is so named because it migrates ahead of albumin in the customary electrophoresis of serum or plasma proteins. It is rarely observed as a distinct band on routine cellulose acetate electrophoretic patterns of serum, although it can be exhibited by high resolution electrophoresis (HRE) or immunoelectrophoresis. It is rich in tryptophan and contains 0.5% carbohydrate. Prealbumin combines with thyroxine and triiodothyronine to serve as the transport mechanism for these thyroid hormones. Prealbumin also binds with retinol-binding protein to form a complex that transports retinol-(vitamin A).

Albumin Albumin is the protein present in highest concentration in the serum . It is synthesized in the liver. The earliest method for its determination involved the salting out of the globulins with sodium sulfate, leaving the albumin in solution. The albumin was then determined by the Kjeldahl method and, later, by the biuret color development. The method commonly used today involves dye binding and the shift in color when a dye is bound by albumin. When more information about proteins is needed, an electrophoretic pattern is obtained, and the albumin is calculated as a percentage of the total protein (usually, approximately 60%). At birth, the reference value for serum albumin averages 39 g/L.

The concentration falls to 28.4 g/L at about 9 months and then begins to increase slowly until adult values of 35-55 g/L are reached. The serum albumin level after age 60 years averages 38.3 g/L.

Albumin has two well-known functions. One is the contribution albumin makes to the colloid osmotic pressure of the intravascular fluid. Because of its high concentration, albumin is responsible for nearly 80% of this pressure, which maintains the appropriate fluid in the tissue. The other prime function is its propensity to bind various substances in the blood.

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