Albumin

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Human albumin is a single non-glycosylated polypeptide with 585 amino acids and a molecular weight of 66,439 Da. Albumin is mainly synthesized in the liver and participates in multiple life activities. It has many important functions, including maintaining oncotic pressure, transporting various endogenous and exogenous ligands, and maintaining plasma pH. The average 70 kg person has approximately 360 g of albumin, which is the most abundant plasma protein, with an average plasma concentration of 42 g / L (632 µM) and a circulating half-life of 19 days. The long-circulating half-life of albumin is the basis of many half-life extension strategies. Albumin is a component of many secretions, including saliva, sweat, tears, and milk. In view of the abundance, stability, long-circulating half-life and inherent binding capacity of albumin, albumin has become an important determining factor in the pharmacokinetics of many drugs and is an excellent drug delivery platform.

Fig 1. Albumin as a versatile drug delivery platform (Sleep, D. 2014)

Ligand binding and allosteric interaction between binding sites

Albumin has an extraordinary ligand binding capacity, which makes it the most important carrier/transporter for endogenous and exogenous ligands in the human body, which increases the circulating half-life of certain drugs and reduces their toxicity and clearance rate. The ligands that bind to albumin include more than thirty different proteins and peptides, as well as various small molecules and metal ions. Bulky heterocyclic anions (such as warfarin) binds to the DIIA core, this site is called the major drug-binding site I or Sudlow’s site I, while aromatic carboxylates (such as ibuprofen and diazepam) are binding to DIIIA, and this site is called the major drug-binding site II, or Sudlow's site II. Most circulating serum albumins have a free sulphydryl group, Cys34. The high reducibility of this sulfhydryl group is attributed to its positioning in the 9.5A˚ deep pocket in DIA and the modulating function of surrounding amino acids both lowering the pKa of the thiol and contributing to its high redox potential. Cys34 acts as the binding site for Au(I), Hg(II) and complex Pt(II), and the natural binding site for nitric oxide (NO), which has been widely used to bind small molecules and protein drugs. Crystallographic studies have identified seven fatty acid binding sites. Under normal physiological conditions, albumin can bind 0.1-2 moles of unesterified fatty acids, but in some disease states, it can bind more.

Fig 2. Biochemical and biophysical properties of albumin for drug delivery (Sleep, D. 2014)

Albumin: a safe and low immunogenic protein

Human albumin is highly polymorphic, and 79 naturally occurring mutations in the coding region of human albumin have been identified. Despite the heterogeneity within the general population, albumin purified from pooled plasma has a long history of safe and successful use, including drug delivery, plasma volume expansion, detoxification, imaging/diagnosis, etc. The immune response to protein therapeutics may be affected by many factors, one result is the production of antidrug antibodies (ADAs) by B-cells. In most biotherapeutic products, the number of ADAs varies, and the consequences are also different, but recombinant albumin is one of the few exceptions.

Surprisingly, human albumin auto-antibodies can be detected in healthy individuals, which are elevated in various disease states including cirrhosis, diabetes, hepatitis, and familial dysautonomia. Bovine serum albumin has 75% homology with human albumin, some human albumin auto-antibodies may actually be directed against bovine serum albumin, which may be produced by ingestion of cow’s milk. Therefore, the identification of certain human albumin autoantibodies may actually be measurement artifacts to assay reagents, or non-specific recognition of HSA. Nevertheless, it seems that the detection of albumin antibodies is a common phenomenon, even if the level is high, it will not lead to immune complex disease, significant decrease in albumin levels or any other directly related clinical condition, even when at highly elevated levels.