albumins

Albumins are a family of soluble, abundant proteins found in animal tissues and fluids. The best known members are serum albumins, such as human serum albumin (HSA) and bovine serum albumin (BSA), and ovalbumin, the main protein of egg white. Serum albumins are secreted mainly by the liver and circulate in plasma as the most abundant plasma protein. They are approximately 66 kDa single-chain polypeptides that remain soluble at physiological pH.

Functions of albumins include maintaining colloid osmotic pressure, which helps retain fluid within the blood vessels. They also serve as versatile transporters for a wide range of ligands, including free fatty acids, bilirubin, calcium and other metal ions, hormones, drugs, and various xenobiotics. Albumins contribute to pH buffering and, in the case of human serum albumin, exhibit antioxidant properties via the thiol group at cysteine-34. Ovalbumin and other egg-white albumins play similar roles in their respective fluids, including nutrient storage and protection of developing embryos.

Synthesis and turnover: Serum albumins are synthesized in hepatocytes in the liver, released into circulation, and have a half-life of about 19–21 days in humans.

Clinical and laboratory relevance: Hypoalbuminemia, due to malnutrition, liver disease, nephrotic syndrome, or inflammatory states, can cause edema and affect drug disposition. Albumin solutions are used therapeutically in some settings to restore plasma oncotic pressure. In the laboratory, BSA and ovalbumin are widely used as standards, controls, and blocking agents.