Plasma proteins
General:
- Prealbumin (transthyretin), 0.1-0.4g/L: Glycoprotein which is usually a faint band on SPEP, but a prominent band on CSF electrophoresis as it is synthesized by the choroid plexus in addition to the liver. It transports thyroxine, triiodothyronine, and in a complex with retinol-binding protein, vitamin A. Its circulation half-life is only ~2 days. Used as a clinical indicator of nutrition status (direct relationship with nitrogen balance), is increased with salicylate therapy, and is a negative acute-phase reactant.
- Albumin, 35-52g/L: The most abundant protein in normal plasma, with numerous functions. Accounts for ~80% of intravascular oncotic pressure/colloid osmotic pressure. Also serves as a source or reserve of amino acids, as a plasma buffer, as an antioxidant by finding free fatty acids, copper ions, & bilirubin, alters capillary permeability by binding certain specific membrane associated glycoproteins to increase permeability to small proteins, and is a carrier protein to numerous other molecules. Synthesized in hepatic parenchymal cells (yolk sac in early fetal development), and has a circulation half-life of 15~19 days. It is elevated in dehydration, decreased in renal (early) and liver (late) disease, malnutrition, burns, & GI diseases, and is a negative acute-phase reactant.
- Alpha-1-globulins:
- Alpha-1-antitrypsin (AAT) is a glycoprotein comprising ~90% of the alpha-1 region of an SPEP and inhibits lysosomal elastase released by reactive neutrophils. It also inhibits chymotrypsin and other trypsin-like enzymes. It is a positive acute-phase reactant.
- Alpha-1-fetoprotein (AFP) is a glycoprotein found in sparse amounts (<2ug/L) in men and non-pregnant women, but is the major protein in fetal serum and is found in large quantities through the first year of life and is increased in pregnant women. If increased beyond the expected level for gestational age, may indicate neural tube defect, GI atresia, multiple gestation, or fetal blood loss. In non-pregnant individuals, increases are associated with hepatitis, primary hepatomas, yolk sac tumors of the ovary, and testicular carcinomas. Down syndrome is associated with decreased AFP.
- Alpha-1-glycoprotein (AGP, orosomucoid) is a glycoprotein made by the liver, granulocytes, and monocytes. Biological function is unclear, though it inhibits phagocytic activity of neutrophils and inhibits platelet aggregation. It also binds propranolol, quinidine, chlorpromazine, and some lipophilic hormones such as progesterone. It is increased in ulcerative colitis, elevated glucocorticoids, and is a positive acute-phase reactant. It is decreased with protein-losing enteropathies and nephrotic syndrome.
- Alpha-2-globulins:
- Alpha-2-macroglobulin (A2M) consists of 4 identical polypeptide chains and is synthesized by hepatocytes, monocytes, and macrophages. It is a protease inhibitor, blocking active sites of plasmin, thrombin, and kallikrein. It complexes with ((PSA|prostate specific antigen); this complex is rapidly cleared. It is increased in nephrotic syndrome.
- Haptoglobin (Hp): A glycoprotein synthesized in the liver which binds to free hemoglobin. Its unbound half-life is ~5 days, but its complex with hemoglobin is cleared within a few hours by the reticuloendothelial system, thereby re-capturing hemoglobin. It is a positive acute-phase reactant, and is decreased with intravascular hemolysis.
- Ceruloplasmin (Cer): is a copper oxidase enzyme made by hepatocytes and consisting of one polypeptide chain carrying 6-8 copper atoms. Has ferroxidase activity, converting iron (Fe+2 -> Fe+3) to the ferric form which can be bound by transferrin. Also acts as an antioxidant, preventing lipid peroxidation & thus cellular damage. Together with albumin, it's involved in copper transport; ~95% of total serum copper is bound to Cer. It is increased in late pregnancy & with estrogen therapy, and is a positive acute-phase reactant. It is decreased in ((Wilson's disease)).
- Beta-globulins:
- Transferrin (Trn) is a glycoprotein made in the liver and which transports oxidized iron (Fe+3). Its half-life is 8~10 days. It has two iron binding sites, the affinity of which are regulated by pH. Normally only ~35% saturated. Total iron binding capacity is an indirect measure (Trn mg/dL = TIBC ug/dL x 0.70). It is increased in iron deficiency anemia, iron overload/hemochromatsis (>70% saturated), in pregnancy, and with estrogen or anabolic steroid use. It is a negative acute phase reactant and is also decreased in renal disease, liver disease, and malnutrition.
- Beta-2-microglobulin (B2M) is a small protein serving as the light or beta chain of HLA on most nucleated cells. Although it filters through the glomerulus, essentially 100% is normally resorbed in the tubules, thus it is a good indicator of renal tubular function. It is also a good indicator for multiple myeloma and other B-cell tumors.
- Fibrinogen is a glycoprotein made in the liver, with a half-life of ~3 days. It is a positive acute-phase reactant, and is also increased in pregnancy, with estrogen therapy, and in renal disease. Decreased levels are associated with consumption such as in disseminated intravascular coagulation. It should NOT be seen on SPEP as it should be removed in clotting to form serum, but in normal plasma is a distinct band between the beta & gamma globulins and may be confused with an abnormal band depending on the sample used.
- Complement includes...
- C-reactive protein ...
- Gamma-globulins, Immunoglobulins (Igs): ...
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