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HSA is used therapeutically as an aqueous solution, and is available in concentrated form (15-25% protein) or as an isotonic solution (4-5% protein). In both cases, in excess of 95% of
Table 9.4. The major plasma proteins of known function found in human blood
Protein Normal plasma Molecular Function
Albumin 35-45 66.5 Osmoregulation transport
Retinol-binding protein 0.03-0.06 21 Retinol transport
Thyroxine binding globulin 0.01-0.02 58 Binds/transports thyroxine
Transcortin 0.03-0.04 52 Cortisol and corticosterone
Caeruloplasmin 0.1-0.6 151 transport
Haptoglobin 1.0-2.2 100 Binds and helps conserve
Type 2-1 1.6-3.0 200 haemoglobin
Type 2-2 1.2-2.6 400
Transferrin 2.0-3.2 76.5 Iron transport
Haemopexin 0.5-1.0 57 Binds haem destined for
b2-microglobulin 0.002 11.8 disposal
Associated with HLA
g-Globulins 7.0-15.0 150 histocompatibility antigen
Transthyretin 0.1-0.4 55 Binds thyroxine
the protein present is albumin. It can be prepared by fractionation from normal plasma or serum, or purified from placentas. The source material must first be screened for the presence of indicator pathogens. After purification, a suitable stabilizer (often sodium caprylate) is added, but no preservative. The solution is then sterilized by filtration and aseptically filled into final sterile containers. The relative heat stability of HSA allows a measure of subsequent heat treatment, which further reduces the risk of accidental transmission of viable pathogens (particularly viruses). This treatment normally entails heating the product to 60°C for 10 h. It is then normally incubated at 30-32°C for a further 14 days and subsequently examined for any signs of microbial growth.
HSA is used as a plasma expander in the treatment of haemorrhage, shock, burns and oedema, as well as being administered to some patients after surgery. For adults, an initial infusion containing at least 25 g of albumin is used. The annual world demand for HSA exceeds 300 tonnes, representing a market value of the order of $1 billion.
Despite screening of raw material and heat treatment of final product, HSA derived from native blood will sometimes (although rarely) harbour pathogens. rDNA technology provides a way of overcoming such concerns and the HSA gene and cDNA have been expressed in a wide variety of microbial systems, including E. coli, Bacillus subtilis, Saccharomyces cerevisiae, Pichia pastoris and Aspergillus niger (its lack of glycosylation renders possible production of native HSA in prokaryotic as well as eukaryotic systems). However, HSA’s relatively large size, as well as the presence of so many disulphide bonds, can complicate recombinant production of high levels of correctly folded products in some production systems. The main stumbling block in replacing native HSA with a recombinant version, however, is an economic one. Unlike most biopharmaceuticals, HSA can be produced in large quantities and inexpensively by direct extraction from its native source. Native HSA currently sells at $2-3/g. Although it can be guaranteed blood pathogen-free, recombinant HSA products will find it difficult to compete with this price.
BLOOD PRODUCTS AND THERAPEUTIC ENZYMES 357
Gelatin is produced by partial acid or partial alkaline hydrolysis of animal collagen. It has a wide variety of therapeutic and pharmaceutical uses. It is often used in the manufacture of hard and soft capsule shells, suppositories and tablets, and is sometimes used as a ‘sponge’ during surgical procedures, as it can absorb many times its own weight of blood.
A 4% solution of gelatin (often modified, i.e. succinylated gelatin) is also used as a plasma expander. For this application, 500ml-1l of the sterile solution is infused slowly. In rare occurrences, infusion (particularly rapid infusion) of the gelatin solution has been known to initiate hypersensitivity reactions. After its infusion, gelatin is excreted relatively quickly and mostly via the urine.
Oxygen-carrying blood substitutes