Human physiology of albumin
Human albumin is a multifunctional transport protein present in the blood that has been widely used in clinical practice for decades in the treatment of volume deficits and conditions exacerbated by hypoalbuminemia
Human albumin is a multifunctional transport protein present in the blood that has been widely used in clinical practice for decades in the treatment of volume deficits and conditions exacerbated by hypoalbuminemia . Because these conditions present unique and heterogeneous pathophysiological features , albumin supplementation is used with different therapeutic objectives for each affected individual and may not always be essential in the context of critical care . 1 Nonetheless , owing to its various physiological and biochemical properties , albumin shows the potential to interact with a number of ligands including pharmacological drugs . 2
Structure and biological properties Structurally , human albumin is a water-soluble , single-peptide chain protein produced in the liver . It consists of 585 amino acids organized into three homologous alpha-helix domains stabilized by disulfide bonds that assume an ellipsoid configuration in solution . 3 Although these domains present a similar structure , they exhibit different ligand-binding affinities , which contribute to the biological versatility of this protein . 3 , 4 Albumin is the most abundant protein in the bloodstream , accounting for 55−60 % of all plasma proteins , although small amounts ( approximately 2g ) can be stored in the liver . 1 In a healthy adult , a total of 12−25g of albumin are produced each day , and content varies from 4 to 5g / kg of body weight , with approximately 40 % seen in the intravascular compartment and the remainder in the interstitial space . 1 A total of 80 % of the extravascular pool is known to reside in the muscle and the skin . 5 The molecular weight ( 66.4 kDa ) of albumin is relatively low compared with that of immunoglobulins and other intravascular proteins . 1 It is a highly stable non-glycosylated negatively charged protein , with a half-life in plasma of approximately 19 days in healthy individuals . 1 , 5 The stability of albumin is due to unique 35 cysteine ( Cys ) amino acid residues , 34 of which are disulfide-bonded ( -S-S ), while the 35th amino acid residue is free at Cys 34 position . 5
Albumin is translated from a single gene in the form of preproalbumin and undergoes cleavage of its prepropeptide terminal in the endoplasmic reticulum of hepatocytes . The mature form of the protein is subsequently formed in the Golgi apparatus and is then released into the intravascular space . 2 , 4 Post-translational modifications include glycation , cysteinylation , S-nitrosylation , S-transnitrosation , and S-guanylation , mainly in free Cys residue 34 , which seem to partly affect the ability of the protein to interact with exogenous molecules in specific environments . 4 Naturally , the most significant factor influencing synthesis is the plasma concentration of albumin itself , but its production can also be induced by changes in interstitial colloid osmotic pressure ( known as oncotic pressure ) as well as changes in the supply of amino acids and energy to the cells , variations in the levels of hormones such as insulin , steroids , and growth hormone , and presence of systemic inflammation . By contrast , albumin catabolism can be more pronounced than synthesis and is essentially regulated by the total albumin pool . 1 , 5
Physiological functions of albumin Table 1 provides a summary of the physiological properties of albumin . 1
Because albumin hardly crosses the majority of capillaries but can pass through fenstrated capillaries , it is responsible for up to 80 % of the plasma oncotic pressure and inhibits the displacement of fluid from the intravascular compartment into the interstitial space and ,
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