Page 285 - Color Atlas of Biochemistry

276 Tissues and organs

Plasma proteins as plasma cells (see p. 302) and peptide hor-
mones, which derive from endocrine gland
Quantitatively, proteins are the most impor- cells.
tant part of the soluble components of the With the exception of albumin, almost all
blood plasma. With concentrations of be- plasma proteins are glycoproteins. They carry
–1
tween 60 and 80 g L ,they constitute ap- oligosaccharides in N-and O-glycosidic bonds
proximately 4% of the body’s total protein. (see p. 44). N-acetylneuraminic acid (sialic
Their tasks include transport, regulation of acid; see p. 38) often occurs as a terminal
the water balance, hemostasis, and defense carbohydrate among sugar residues.
against pathogens. Neuraminidases (sialidases) on the surface of
the vascular endothelia gradually cleave the
sialic acid residues and thereby release ga-
A. Plasma proteins
lactoseunits on thesurfaces ofthe proteins.
Some 100 different proteins occur in human These asialoglycoproteins (“asialo-” = without
bloodplasma. Basedon their behavior during sialic acid) are recognized and bound by gal-
electrophoresis (see below), they are broadly actose receptors on hepatocytes. In this way,
divided into five fractions: albumins and α 1 –, the liver takes up aged plasma proteins by
α 2 –, β–and γ-globulins. Historically, the dis- endocytosis and breaks them down. The oli-
tinction between the albumins and globulins gosaccharides on the protein surfaces thus
was based on differences in the proteins’ determine the half-life of plasma proteins,
solubility –albumins are soluble in pure which is a period of days to weeks.
water, whereas globulins only dissolve in In healthy individuals, the concentration of
thepresenceof salts. plasma proteins is constant. Diseases in or-
The most frequent protein in the plasma, at gans that are involved in protein synthesis
–1
around 45 g L ,is albumin. Due to its high and breakdown can shift the protein pattern.
concentration, it plays a crucial role in main- For example, via cytokines (see p. 392), se-
taining the blood’s colloid osmotic pressure vere injuries trigger increased synthesis of
and represents an important amino acid re- acute-phase proteins, which include C-reac-
serve for the body. Albumin has binding sites tive protein, haptoglobin, fibrinogen, comple-
forapolarsubstancesand thereforefunctions ment factor C-3, and others. The concentra-
as a transport protein for long-chain fatty tions of individual proteins are altered in
acids, bilirubin, drugs, and some steroid hor- some diseases (known as dysproteinemias).
mones and vitamins. In addition, serum albu-
min binds Ca 2+ and Mg 2+ ions. It is the only
important plasma protein that is not glycosy- B. Carrier electrophoresis
lated. Proteins and other electrically charged mac-
The albumin fraction also includes trans- romolecules can be separated using electro-
thyretin (prealbumin), which together with phoresis (see also pp. 78, 262). Among the
other proteins transports the hormone thy- various procedures used, carrier electropho-
roxine and its metabolites. resis on cellulose acetate foil (CAF) is partic-
The table also lists important globulins in ularly simple. Using this method, serum pro-
blood plasma, with their mass and function. teins—which at slightly alkaline pH values all
The α-and β-globulins are involved in the move towards the anode, due to their excess
transport of lipids (lipoproteins; see p. 278), of negative charges—can be separated into
hormones, vitamins, and metal ions. In addi- the five fractions mentioned. After the pro-
tion, they provide coagulation factors, pro- teins have been stained with dyes, the result-
tease inhibitors, and the proteins of the com- ing bands can be quantitatively assessed us-
plement system (see p. 298). Soluble antibod- ing densitometry.
ies (immunoglobulins; see p. 300) make up
the γ-globulin fraction.
Synthesis and degradation. Most plasma
proteins are synthesized by the liver. Excep-
tions to this include the immunoglobulins,
which are secreted by B lymphocytes known

Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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