Human serum albumin (HSA), the most abundant protein in human blood plasma, is essential for maintaining osmotic pressure. It is produced in the liver, consists of a single polypeptide chain, with 585 amino acids with 17 tyrosil residues and one tryptophan located in position 214. HSA is organized in three domains, I, II and III, each consisting of two subdomains, A and B. In the physiological states, HSA occurs in two forms – the non-modified HSA with a free thiol group of Cys-34, and the modified (oxidized) form (oHSA), also called human mercaptoalbumin (HMA) and human nonmercaptoalbumin (HNA), respectively. HMA and HNA are in equilibrium, depending on the redox state of Cys-34, and their ratio also varies depending on age and the diseased state. HSA functions primarily as a carrier protein for drugs, steroids, fatty acids, and thyroid hormones, and plays a role in stabilizing extracellular fluid volume.

> 95% as determined by SDS-PAGE