Albumins
Albumins
Definition and Sources
Albumins (from Latin “albumen” meaning protein) are simple globulins, low molecular weight proteins found in human and animal fluids and certain plant seeds. Examples include serum albumin, milk lactalbumin, ovalbumin, ricin, leukocin, and legumin (albumin of leguminous plants). They account for up to 55-60% of human blood plasma proteins.
Properties and Functions
Albumins are water-soluble and easily crystallize in a saturated Na2SO4 solution. They bind water, metal ions, and other substances. The concentration of albumins in blood serum ranges from 46-50 g/l, and about 12 g are synthesized in the liver daily. Their half-life (T1/2) is about 20 days. The structure includes 585 amino acid residues and 17 disulfide bonds. Albumins have a molecular weight of 35,000-70,000 Da and an isoelectric point (pI) of 4.7. They consist of a single polypeptide chain, with aspartic acid at the N-terminus and alanine at the C-terminus. Albumins are unique among proteins as they are not glycoproteins.
Albumin Molecule Characteristics
Binding Properties
The albumin molecule contains many dicarboxylic amino acids (glutamic, aspartic), allowing it to retain Ca2+, Cu2+, and Zn2+ cations in the blood. Rich in lysine, leucine, and valine, albumins have minimal amounts of tryptophan, cysteine, and methionine. Their negative electrical charge is higher than that of globulins, so they move faster towards the anode. Albumins are the most homogeneous fraction of blood plasma proteins.
Distribution and Function
About 40% of albumins circulate in the blood, while the rest are in the interstitial fluid, which has a volume 4 times larger than the blood plasma volume. Due to their relatively low molecular weight and high concentration, albumins account for up to 80% of blood plasma osmotic pressure. There are about 310-330 g of albumins in the human body, with 110-130 g in the blood. In cases of hypoalbuminemia, the osmotic pressure of blood plasma decreases, disrupting the fluid balance between blood vessels and the interstitial space.
Pathological Conditions
Hypoalbuminemia and Hyperglobulinemia
Hypoalbuminemia can result from decreased albumin synthesis during liver diseases (e.g., cirrhosis), increased capillary permeability, protein loss due to burns, catabolic states (e.g., sepsis, malignant tumors), nephrotic syndrome accompanied by albuminuria, and fasting. Hyperproteinemia is most often accompanied by hyperglobulinemia, while hypoproteinemia is accompanied by hypoalbuminemia. The decrease in blood plasma volume reduces renal blood flow, activating the renin-angiotensin-aldosterone system, which strives to restore blood volume. However, without sufficient albumins to retain Na+, other cations, and water, fluid migrates to the interstitial space, causing edema.
Transport Functions
Albumins play a crucial role in transport, carrying free fatty acids, unconjugated bilirubin, triiodothyronine, and many drugs (e.g., aspirin, dicumarol, sulfonamides). During the treatment of diseases causing hypoalbuminemia, it is important to remember that the amount of free drugs in the blood increases. Certain drugs may also compete for binding sites in albumins with bilirubin or each other.
Prealbumins and Transthyretin
Prealbumins are the most mobile fraction of blood plasma during electrophoresis, accounting for 0.2-0.3% of all blood proteins. Synthesized in the liver, their primary function is binding thyroxine and forming complexes with retinol-binding protein.
Transthyretin
Transthyretin, a prealbumin, binds thyroxine and belongs to the albumin fraction. It is an acute phase protein and can bind retinol-binding protein in one site and up to two molecules of thyroxine and triiodothyronine in another.
Source | Glossary of Most Commonly Used Biomedical Terms and Concepts | Lithuanian University of Health Sciences | Academician Professor Antanas Praškevičius, Professor Laima Ivanovienė