Biomedical Engineering Reference
In-Depth Information
quantities required by patients heightens the risk of accidental transmission of blood-borne patho-
gens. The α
1
-antitrypsin gene has been expressed in a number of recombinant systems, including
in the milk of transgenic sheep. Although use of the recombinant product would all but preclude
blood pathogen transmission, it appears signifi cantly more costly to produce.
12.4.8 Albumin
HSA is the single most abundant protein in blood (Table 12.7). Its normal concentration is ap-
proximately 42 g l
1
, representing 60 per cent of total plasma protein. The vascular system of an
average adult thus contains in the region of 150 g of albumin. HSA is responsible for over 80 per
cent of the colloidal osmotic pressure of human blood. More than any other plasma constituent,
HSA is thus responsible for retaining suffi cient fl uid within blood vessels. It has been aptly de-
scribed as the protein that makes blood thicker than water.
Albumin molecules also temporarily leave the circulation, entering the lymphatic system, which
harbours a large pool of this protein (up to 230 g in an adult). Lower quantities of albumin are also
present in the skin.
In addition to its osmoregulatory function, HSA serves a transport function. Various metabo-
lites travel throughout the vascular system predominantly bound to HSA. These include fatty
acids, amino acids, steroid hormones and heavy metals (e.g. copper and zinc), as well as many
drugs.
HSA is a 585 amino acid, 65.5 kDa polypeptide. It is one of the few plasma proteins that are
unglycosylated. A prominent feature is the presence of 17 disulfi de bonds, which help stabilize the
molecule's three-dimensional structure. HSA is synthesized and secreted from the liver, and its
gene is present on human chromosome number 4.
Table 12.7
The major plasma proteins of known function found in human blood
Normal plasma
concentration (g l
1
)
Molecular
mass (kDa)
Protein
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 transport
Ceruloplasmin
0.1-0.6
151
Copper transport
Haptoglobin
Binds and helps conserve haemoglobin
type 1-1
1.0-2.2
100
type 2-1
1.6-3.0
200
type 2-2
1.2-2.6
400
Transferrin
2.0-3.2
76.5
Iron transport
Hemopexin
0.5-1.0
57
Binds haem destined for disposal
β2-Microglobulin
0.002
11.8
Associated with human leukocyte antigen
histocompatibility antigen
γ-Globulins
7.0-15.0
150
Antibodies
Transthyretin
0.1-0.4
55
Binds thyroxine
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