Plasma proteins, also termed serum proteins or blood proteins, are proteins present in blood plasma. They serve many different functions, including transport of lipids, hormones, vitamins and metals in the circulatory system and the regulation of acellular activity and functioning and in the immune system. Other blood proteins act as enzymes, complement components, protease inhibitors or kinin precursors. Contrary to popular belief, hemoglobin is not a blood protein, as it is carried within red blood cells, rather than in the blood serum.
Serum albumin accounts for 55% of blood proteins, and is a major contributor to maintaining the osmotic pressure of plasma to assist in the transport of lipids and steroid hormones. Globulins make up 38% of blood proteins and transport ions, hormones and lipids assisting in immune function. Fibrinogen comprise 7% of blood proteins; conversion of fibrinogen to insoluble fibrin is essential for blood clotting. The remainder of plasma proteins (1%) is made up of regulatory proteins such as enzymes, proenzymes and hormones. All blood proteins are synthesized in liver except for the gamma globulins.
Separating serum proteins by electrophoresis is a valuable diagnostic tool as well as a way to monitor clinical progress. Current research regarding blood plasma proteins is centered on performing proteomics analyses of serum/plasma in the search for biomarkers. These efforts started with two-dimensional gel electrophoresis efforts in the 1970s and in more recent times this research has been performed using LC-tandem MS based proteomics. The normal laboratory value of serum total protein is around 7 g/dL.
Families of blood proteins:-
|Blood protein||Normal level||%||Function|
|Albumins||3.5-5.0 g/dl||55%||maintains colloid osmotic pressure; create oncotic pressure and transport insoluble molecules|
|Globulins||2.0-2.5 g/dl||38%||participate in immune system|
|Fibrinogen||0.2-0.45 g/dl||7%||Blood coagulation|
|Regulatory proteins||<1%||Regulation of gene expression|
|Clotting factors||<1%||Conversion of fibrinogen into Fibrin|
Examples of specific blood proteins:-
- Alpha 1 antitrypsin (neutralizes trypsin that has leaked from the digestive system)
- Alpha 1 acid glycoprotein
- Alpha 1 fetoprotein
- Gamma globulins
- Beta 2 microglobulin
- Complement component 3
- Complement component 4
- C-reactive protein (CRP)
- Lipoproteins (chylomicrons, VLDL, LDL, HDL)
- MBL or MBP
Clinical Chemistry : a laboratory perspective / [edited by] Wendy Arneson, Jean Brickell.