Albumin - The Importance of Quality Protein in Animal Feed
Written by: Dr Tom Shurlock for and on behalf of GWF Nutrition Limited.
Copyright GWF Nutrition Limited May 2020 - Not for reproduction.
The albumins are a family of globular proteins which include serum albumin, ovalbumin, lactalbumin (storage proteins of egg and milk) and seed proteins, of which hemp is a particularly rich source.
Albumin is the main circulating protein in the body; it accounts for ~ 50% of plasma protein. It is a relatively small molecule being a single chain of 585 amino acids. It contains 35 cysteine molecules, 34 of which pair off with disulphide bonds which stabilise the shape into a globular, heart-shaped conformation.
They are organized in three repeated homologue domains (sites I, II, and III), each of which comprised of two separate sub-domains (A and B). This conformation enables albumin to bind a range of cations and hydrophobic constituents, thus enhancing their transport in a water-based medium.
Structure of serum albumin, displaying domains.
Amino acid sequence of Albumin, showing CCx..xC repeat that shapes molecule.
Synthesis of albumin occurs in the liver, the molecule having a relatively short half-life in its circulatory phase, and so needs constant renewal. Serum albumin has an oncotic function (drawing water into the circulatory system to maintain osmotic pressure), but it also has a number of other functions elated to its binding characteristics.
The sulphur links between the cysteine molecules – thiols - act as scavengers of reactive oxygen species (ROS). Additionally, the N-terminal end of the molecule can bind with cations (Cu, Co, Zn etc.) which inhibits the actions of them to generate free radicals (Caraceni et al., 2013).
Albumin appears to inhibit the secretion of pro-inflammatory cytokines (TNF-α) and complement factors (C5a) through the modulation of the signalling systems between inflammatory cells, such as neutrophils and endothelial cells (Chen et al., 2009; Kitano et al., 1996).
A large number of binding sites with different affinities for plasmatic compounds have been identified: the major domains are capable of folding into hydrophobic pockets, which can open and close, and accommodate large insoluble anions; furthermore, cationic groups located on the surface of the molecule allow the formation of ionic bonds with many different ligands.
As a result, HSA binds and carries a great variety of hydrophobic molecules, such as endogenous (i.e., cholesterol, fatty acids, bilirubin, thyroxine) or exogenous substances (i.e., drugs), transition metal ions, and gas (nitric oxide [NO]), with consequent implications on their solubilization, transport and metabolism.
As illustrated by the interaction of vitamin D binding protein with cholecalciferol, albumin can bind with liposomes, via the thiol links, to create an antibody response to entrapped antigens (Shek & Heath, 1983).
Albumin’s ability to bind hydrophobic molecules in an aqueous environment does impact on the absorptive process, as it carries nutrients from a lipid surface (the intestinal cell walls) to the surface of target cells. Ingested nutrients, similarly bound to dietary albumins, can deliver aforementioned compounds to intestinal cell walls and so aid both active and passive transport.
A specialised form of albumin is the vitamin D binding protein (DBR). As with serum albumin, the binding is characterised by the shape, fixed by the sulphur links of 28 cysteine molecules. These enable the folding over the hydrophobic core of cholecalciferol. Fatty acids are also bound into these complexes. It is thought that the action of binding of hydrophobic moieties alter the conformation of DBP and albumins generally, which enable their release at the target site, possibly by pinocytosis (Kunwar et al., 2006).
Dietary sources of albumin include seed proteins but hemp is a particularly rich source. A third of hemp protein is albumin, high in sulphur amino acids, although there is little data on the thiol induced shape of the molecule. However, it is likely its shape can transport hydrophobic molecules in the gut and so enhance the bioavailability of substances such as curcumin; although curcumin transport is more favoured as a liposome, albumin has also been shown as a powerful transporter (Kunwar, 2006).
Notwithstanding, hemp albumin will be enzymatically broken down in the small intestine, absorbed and transported to the liver. Within these moieties are bioactive peptides. These are usually sequences of 4-05 amino acids and will therefore encompass the cysteine/thiol links that are characteristic of albumins. As such, they will help drive the genesis of serum albumin. Biochemical reactions depend on supply and demand and so a build-up of thiol links should drive serum albumin levels and associated improvement of hydrophobic transport. This in turn increases the bio-availability of certain products.
Hemp protein has a higher albumin, thiol content and soluble protein than, for example, soya protein, and also displays greater digestibility (Wang, 2008`. This makes it an ideal protein source to help transport hydrophobic substances to the gut wall and, on dissemination, provides the base for synthesising serum albumins that transport materials to target cells.
Caraceni P, Domenicali M, Tovoli A, Napoli L, Ricci CS, Tufoni C, Bernardi M. Clinical indications for the albumin use: Still a controversial issue. European Journal of Internal Medicine 24 (2013) 721–728
] Chen T-A, Tsao Y-C, Chen A, Lo G-H, Lin C-K, Yu H-C, et al. Effect of intravenous albumin on endotoxin removal, cytokines, and nitric oxide production in patients with cirrhosis and spontaneous bacterial peritonitis. Scand J Gastroenterol 2009;44:619–25
Kitano H, Fukui H, Okamoto Y, Kikuchi E, Matsumoto M, Kikukawa M, et al. Role of albumin and high-density lipoprotein as endotoxin-binding proteins in rats with acute and chronic alcohol loading. Alcohol Clin Exp Res 1996;20:73A–6A
Kunwar A, Barik A, Pandey R, Priyadarsini KI. Transport of liposomal and albumin loaded curcumin to living cells: An absorption and fluorescence spectroscopic study. Biochimica et Biophysica Acta 1760 (2006) 1513–1520
Qiao R, Siflinger-Birnboim A, Lum H, Tiruppathi C, Malik AB. Albumin and Ricinus communis agglutinin decrease endothelial permeability via interactions with matrix. Am J Physiol 1993;265:C439–46.
SHEK PN, HEATH TD. Immune response mediated by liposome-associated protein antigens. III. IMMUNOGENICITY OF BOVINE SERUM ALBUMIN COVALENTLY COUPLED TO VESICLE SURFACE. Immunology 1983 50 101
Speeckaert M, Huang G, Delanghe JR, Taes YE. Biological and clinical aspects of the vitamin D binding protein. Clinica Chimica Acta 372 (2006) 33–42
Wang X, Tang C, Yang X, Gao W. Characterisation, amino acid composition and in vitro digestibility of hemp (Cannabis sativa L.) proteins. Food Chemistry 107 (2008). 11-18.