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Connective tissue - extracelular
matrix
Collagen anad elastin – structure and biosynthesis
Proteoglycans, fibronectin and laminin
Bone tissue.
2009 (J.S.+ E.T.)
Connective tissue
• The main component is extracelular matrix
 determines mechanic properties of tissue
• Proportion of cells is low.
2
Cells in the connestive tissue
• fibroblasts and fibrocytes (chondroblasts and
chondrocytes, osteoblasts and osteocytes) produce the
major components of extracellular matrix,
• histiocytes (fixed and free macrophages) scavenge foreign
particles and tissue detritus,
• mast cells (mastocytes) releasing heparin and histamine,
• adipose cells (adipocytes),
• leukocytes, namely lymphocytes, plasmocytes, and
monocytes (circulating macrophages, leaving vessels
through diapedesis
3
Etracellular matrix
• Fibrilar components – fibrils of collagen and reticulin,in
some connective tissue types elastin fibres, and
• Interfibrillar component (the amorphous ground
substance)
proteoglycans and hyaluronic acid,
fibronectin, laminin and other cell adhesion
glycoproteins,
in bones, voluminous insoluble mineral
component.
4
Main components of extracelullar matrix
Cell cytoskeleton
receptor
Fibronectine
Colagen
fibrils
Elastic fibers
Hyaluronic
acid
Meisenberg, Simmons:Principles of Medical Biochemistry
proteoglycanes
5
Function of extracelular matrix
•Fills the spaces between cells, binds cells and tissue together
Supporting function for cells
Regulation: of polarity of the cells
cell division
adhesion
motion
Grow and regeneration of tissues
Determining the shape of tissue
Architecture of tissue and organs
Membrane filtration barrier
Exchange of metabolites, ions and water
6
Collagen
• The most abundant protein in the body (25% of proteins in
adults)
• Present in all connective tissues
• Responsible for strength and flexibility
• 27 (?) types of collagen known in human – they differ by
structure and function
• Only some types are fibilar (I, II, III, V,VI, XI )
• In all collagen types is present (at least partially) typical
triplhelix
7
Approximate collagen content in various
tissues
Tissue
Demineralized bone
Tendons
Skin
Cartilage
Arteries
Lung
Liver
Collagen content %
90
80-90
50-70
50-70
10-25
10
4
8
Amino acids in collagen
• Fibrilar collagens (-chains)  polymer of repeats GlyX-Y
where:
in position X is very often proline, 3-hydroxyproline, Glu,
His, Leu, Phe,
in position Y 4-hydroxyproline, Thr, Lys, Arg.
• At each third position is glycin, in average each forth
amino acid is proline or hydroxyproline
• The content of glycine 30%, hydroxyproline and
hydroxylysine 25%
9
-helix
Secondary structure of chains
High content of proline and hydroxyproline 
the chain forms polyproline type helix (the
chain cannot turn to be stabilized by hydrogen
bonds between C=O and NH of peptide bond
unlike the -helix).
Secondary structure is an extended helix left –
handed helix with 3 amino acids per turn.
Secondary structure is stabilized by formation
of trimers at which three helical peptides are
wound around each other in right-handed triple
helix – structure is held together by hydrogen
bonds between peptide bonds of interacting
polypeptides
10
Tropocollagen structure
• The basic structural unit of collagen is triple helix
300 x 1,5 nm
•
in collagen type I
two chains 1(I) and one chain 2(I)
(structural formula [1(I)]22(I))
11
Structure tropocollagen
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Glycosylation of collagen
• Hydroxy groups of hydroxylysine are modified by
posttranslational (cotranslational) glycosylation
• Most often glucose a galactose
13
Chain types in collagen
• Amino acid composition of polypeptide chains is variable
• About 30 separarate genes are known
• Best described and mostl wide-spread are 1 and 2
chains
14
Collagen types
• Fibril-forming collagens - type I,II,III, V, XI (90% of all
collagens)
• Network forming collagens – type IV, VIII, X
• FACIT*s– type IX,XII, XIV, XVI, XIX
• Transmembrane collagens – XIII, XVIII
• Others…..
*fibril associated collagens with interupted triple helices
15
Some structural divergences in collagen types I, II, III
Collagen I – the most common type
(skin, bones, tendons, dentin), pevný
v tahu. Slightly glycosylated (~1
%); no cysteinyl residues.
Collagen II – the main type in the
hlavní typ hyalinne cartilage of
joints. High degree of
glycosylation, no cysteinyl residues.
Collagen III (skin, aorta,
uterus)elastic type, forms the thin
reticuline fibrils. Very low
glycosylation, cysteinyl residue are
present, small number of disulfide
bridges.
16
Collagen IV
• The main componet of basement membranes ( e.g. renal
glomeruli, capsule of eye lens).
• do not form fibrils – its flexible triple helices include some
non-helical segments and at their C-ends there are globular
domains,
• chains associate into the networks
• Saccharidic component about 15 %, cysteinyl residues and
disulfide bridges are present.
Globular domain
17
Collagen IV
Association of monomers by
carboxy terminals interaction –
formation of dimers
Asociation of four dimers at
the amino terminal domains
Formation of lattice
from tetramers
18
Synthesis of collagen
Fibroblasts, chondroblasts and osteoblasts
• First phase has intracellular location:
The synthesis of separate procollagen chains (pro-1, pro-2, etc.)
Cotranslational hydroxylation of Pro a Lys
and glycosylation of hydroxylysine (Hyl) in ER
Formation of procollagen from three chains
Secretion of procollagen by exocytosis
• Second phase is located extracellulary
Removal of N- and C-terminal propeptides – formation of tropocollagen
Agregation of tropocollagen units – formation of protofibrils
Interaction of protofibril with proteoglycans – formation of microfibrils
Collagen ripening – oxidation of lysine side chains to allysine, formation of
intermolecular links
19
Details of collagen biosynthesis:
Synthesis of chains and glycosylation
Ribosomal synthesis of separate procollagen chains (pro-1,
pro-2, etc.), the chains enter the endoplasmic reticulum.
Removal of leader sequence  formation of procollagen
chains
Cotranslational hydroxylation of Pro a Lys – monooxygenase
reaction
Glycosylation of 5-hydroxylysin (Hyl) - glycosyltransferases
20
Hydroxylation of proline and lysine residues
monooxygenáza
(askorbate, Fe2+)
CO2
proline
2-oxoglutarate
4-hydroxyproline
succinate
5-hydroxylation lysine residues is analogical
For both hydroxylation reaction ascorbate is
necessary
21
Scurvy
Ascorbic acid deficiency  hydroxylations do not proceed
Hydroxyproline residues are required for stabilization of
triple helix by hydrogen bond formation
Melting temperature of collagen can drop from 420 to 240
Most part of abnormal collagen is degraded in the cell
Subcutaneous hemorrages, bleeding gums, poor
wound healing etc.
22
Iniciation of triple helix formation
Extension sequencies – sequencies of 20-35 AA on both
amino- and carboxy- terminals with high content of
cysteine residues
Formation of procollagen triple-helix is initiated by
formation of interchain disulfide bridges on C-terminal
peptides – self-assembly
C-terminal of
procollagen
Disulfide bridges
between chains in
N-terminal "extension“
parts of procollagen
Three pro--chains
(hydroxylated Pro
and glycosylated Hyl)
„Extension“ C-terminal
parts of procollagen
23
Winding of the procollagen middle parts results in the
triple helix
Molecules of tropocollagen are translocated to Golgi
apparatus, formation of secretory vesicles
24
Procollagen secretion
• Secretory vesicles fuses with the membrane and releases
procollagen into the extracellular space
• Only triple helices can be secreted
• Improperly coiled molecules are degraded
25
Conversion of procollagen to tropocollagen
• Specifické procollagenpeptidases catalyze the removal of
non-helical pro-peptides on both ends
• (In clinical biochemistry, the abbreviations PINP and PICP).
procollagen I N-terminal propeptide
tropocollagen
procollagen I C-terminal propeptide
26
Spontaneous association of tropocollagen units –
- formation of protofibrils
Bundles with diameter that corresponds to about 5 – 10 units.
units in protofibrils are shifted by ¼ of their length, so that the
regularity of gaps and overlaps generates the banding pattern in the
fibrils
N-terminals
C-terminals
67 nm
27
Association of protofibrils with proteoglycans
Protofibrils of collagen interact with
glycosaminoglycan chains of
proteoglycans:
Proteoglycan
Collagen protofibrils
Chondroitin or
keratan sulfates
Core protein
Bundles or felty tangles of protofibrils
of fibrillar collagen types compose
thick microfibrils.
Hyaluronic acid
28
Maturing of collagen – formation of covalent
crosslinks between the chains
Oxidative deamination of lysine an nonglycated residues of hydroxylysine
H2N CH2 CH2
C O
CH2 CH2 C
NH
Lysine residue tropocollagen chain
O2
Lysyl oxidase
(Cu2+-enzyme)
Allysine
O CH CH2
C O
NH3+H2O
CH 2 CH2 C
NH
29
Interchain covalent crosslinks
originate in non-catalyzed reactions between the side chains of allysine and lysine
residues.
Simple covalent bridges join only two adjacent polypeptide chains,
two types of those crosslinks are possible:
– Aldimine type, when the aldehyde group of allysine reacts with the -amino
group of lysine. The product is an aldimine (Schiff base):
–CH2-NH2 + O=CH-CH2–
–CH2-N=CH-CH2–
(+ 2H  –CH2-NH-CH2-CH2–)
The reaction is rapid, but the product unstable. It is stabilized very slowly by
hydrogenation, the crosslink is then called lysinonorleucine bridge.
– Aldol type, when two aldehyde groups of allysine react with each other
(aldol condensation). One aldehyde group remains free in the bridge, so
that it can take part in another reaction.
–CH2-CH=O + O=CH-CH2–
–CH-CH(OH)-CH2–
CH=O
( –C=CH-CH2– + H2O)
CH=O
The resulting aldol is unstable, but it can be stabilized by elimination of water (a
double bond is formed) to the dehydroallysinealdehyde bridge..
30
Interchain covalent crosslinks
31
Hydroxypyridinium type bridges.
NH-CH-CO
CH2
HO
N
NH
CH2-CH2 CH
CO
+
CH2-CH2-CH2
CH2
NH-CH-CO
Aldimine bridges can react with
the aldehyde group of another
allysine side chain
The heterocyclic pyridinium ring
is closed that binds three
polypeptide chains together
through stable covalent bonds –
the hydroxypyridinium type of
crosslink is formed..
The hydroxypyridine ring is a very stable structure. It can be determined
even after hydrolysis of all peptide bonds in collagen fragments, e.g.,
excreted into the urine, as deoxypyridinoline or pyridinoline.
32
Hydroxypyridinium bridge
33
Structure of collagen fibrils
electron optical picture
of soft connective tissue
1 m
0,3 - 3 m
protofibrils are formed through
aggregation of tropocollagen
units
Tropocollagen unit
microfibrils – either a felty tangle or
arranged bundles of protofibrils
interacting with proteoglycans
10 - 30 nm
1,5 nm
34
Collagen degradation
• Molecules of collagen are metabolically very stable, half-life
may be several years.
• Their breakdown is increased during starvation or
inflammatory diseases.
• Degradation by collagenases*
*collagenases – enzymes from the family of matrix
metaloproteinases (MMP). Proteinases containing Zn, that is
necessary for their proteolytic function.
35
Collagen and diseases of connective tissue
•
Fibroses - overproduction of collagen
- lung fibrosis
- liver cirhosis
- atherosclerosis
•
Inherited abnormalities in collagen structure
Osteogenesis imperfecta (brittle bones)
mutation in the gene for 1 and 2 chains of collagen I – impaired
formation of triple helices of collagen collagen fall prey to
intracellular proteases
Ehlers-Danlos syndrom
group of diseses, at least 10 types
characterized by stretchy skin and loose joints („india rubber“ man)
impairment of collagen synthesis due to mutation of genes
hyperexensibility of the skin, abnormal tissue fragility, increased joint
mobility, „cigarette paper“ scars
36
Elastic fibres
• components of elastic connective tissue and elastic
cartilage
• arterial walls, pulmonary alveoli, skin, ligamentum
nuchae
• composed of elastin surrounded by a microfibrillar
sheath that consists of glycoprotein fibrilin and
fibromodulin.
During the formation of elastic fibres, those two proteins are
arranged in the form of oxytalan fibrils, which serve as a mould
that is then filled by tropoelastin, and in which tropoelastin is
crosslinked to mature elastin.
37
Elastin
• Produced by smooth muscle cells, fibroblasts and
chondrocytes
• Probably only one genetic type of elastin, variants arise by
alternative splicing
• Amorphous structure with large amount of crosslinks, non
soluble.
• The main amino acids are glycine (31%), proline (11%),
valine, alanine (22%) and other hydrophobic AA
• Only small portion of hydroxyproline (1%)
• No glycosylation
• Chains don't have a regular secondary structure
• Precursor in the synthesis is tropoelastin
38
Covalent crosslinks in elastin
Derived from allysine
Simple bridges between two residues- aldol or aldimine type
Merodesmosine - three side chains (the reaction of aldol with the lysine
side chain) - acyclic.
Four side chains of allysine and lysine form a tetrafunctional crosslink
(the reaction of merodesmosine with allysine) with the stable pyridinium
structure called either desmosine or isodesmosine:
CH2-CH2-CH2
CH2-CH2
CH2-CH2
CH2-CH2
CH2-CH2
+
N
N+
CH2
CH2
CH2-CH2-CH2
desmosine
CH2-CH2-CH2
CH2-CH2-CH2
isodesmosine
After the maturation of crosslinks elastine become highly insoluble, extremly
39
stable and has very low turnover rate.
Elasticity of elastin
During relaxation – compact disordered structure, hydrophobic
interactions between the chains.
Strech loosens these interactions, while the elastin network is still held
together by covalent crosslinks
tah
uvolnění
40
Synthesis, zrání and maturation of elastin
The synthesis of elastin occurs only in the last phase of foetal development, it
is completed practically soon after the childbirth. The later synthesis of
considerable amounts of elastin is not supposed.
INTRACELLULAR synthesis in fibrocytes
and leiomyocytes of blood vessel walls
PROELASTIN
Peptide fragments ?
posttranslational processing
(incl. limited proline hydroxylation)
soluble TROPOELASTIN
SECRETION OF TROPOELASTIN into the
EXTRACELLULAR matrix
partial degradation
of tropoelastin chains
insoluble ELASTIN
incorporation into the fibrilin mould,
spatial arrangement (coacervation),
allysine is formed (lysyloxidase),
non-catalyzed formation of desmosines
(crosslinking), interactions with proteoglycans
a very long biological half-life
41
Degradation of elastin
specific elastases
antielastase factors
supporters of breakdown
ozone, (NO)x, tobacco smoke,
in vessels – intercalation of LDL,
bile and fatty acids
α1-antitrypsin,
α2-macroglobulin,
HDL in blood vessels
peptide fragments
(some of them are potent immunogens)
42
Elastin exhibits a very long biological half-life (the half-life of elastin in the wall of
aorta is referred to be about 40 years). After the childbirth, the synthesis of elastin is
distinctly reduced, if it is possible at all.
The loss of tissue elasticity
(namely of skin, large vessels, lung) in aging organisms is indisputable. Much
attention is paid to the elastin degradation.
Wrinkles are the sign of low skin elasticity that decreases by 2 - 5 % per decade
(extrapolated to neonatal period). Wrinkles represent rather a cosmetic problem,
though it is extremely interesting from the commercial point of view to those who
produce cosmetics and feign the knowledge how to remove wrinkles.
The loss of vessel wall elasticity in ageing doesn't quite depend on the low
elastin content, but rather on elastin modification due to partial degradation caused
by activity of elastase (released mostly by polymorphonuclear leukocytes),
supported by the interactions of elastin fibres with LDL or bile acids.
The loss of lung tissue elasticity is associated directly with the degradation of
elastin in the walls of alveoli. Elastase is released from circulating neutrophils but,
under normal conditions, elastase is nearly completely inhibited by α1-antitrypsin.
Inherited deficit of 1-antitrypsin or factors that diminish its effect (smoking, ozone in
the ground smog) distinctly increase the risk of pulmonary emphysema.
43
Fibrilin
•The main component of microfibrils (oxytalan and
elastic fibres)
• large glycoprotein
•Secreted by fibroblasts, becomes incorporated into
the insoluble microfibrils
• Marfan syndrom : mutation of fibrilin gen
fibrilin is deffectiveabnormalities in in
connective tissue
it affects the eyes (dislocation of the lens), the skeletal system (most
patients are tall, exhibit long digits) cardiovascular system (weakness
of the aortic media, dilatation of ascending aorta)
44
Proteoglycans
• The main component of amorphous ground substance
• glycosaminoglycans connected with core protein
„bottle-brush“
Chains of GAG
Core protein
N-end
45
Function of proteoglycans and
glycosaminoglycans
• Formation of cell-free ground substance
–
–
–
–
Pressure resistance
Shape recovery
Lubrication of joints
Hydration of cartilages
• binding plenty of water
• structural organization of extracellular matrix
- formation of networks
- binding of Ca2+ in bones
• Cell adhesion and migration
• Enzyme regulation, membrane receptors
46
Glycosaminoglycans (GAGs)
• unbranched acidic polysaccharides that consist of repeating
disaccharide units
[ glycosamin – uronic acid]n
Glukosamine, galactosamine
(often acetylated)
Glucuronic, galacturonic,
iduronic
Specific –OH groups of saccharides may be sulfated
47
Main types of glycosaminoglycans
Heteroglycan
Složení
• hyaluronic acid
• Glc-NAc, Glc-UA
• chondroitin-4-sulfate
• Gal-NAc-4-sulfate, Glc-UA
• chondroitin-6-sulfate
• Gal-NAc-6-sulfate, Glc-UA,
• keratan sulfate
• Gal-NAc, sulfate
• heparin
• Glc-NAc, Glc/Ido-UA, sulfate
• dermatan sulfate
• Gal-NAc, Glc/Ido-UA, sulfate
48
Occurence of GAG
Type of GAG
occurence
Hyaluronic acid
Embryonal tissues, sinovial fluid, eye,
cartilage, loose connective tissue
Dermatan sulfate
skin, vessels, heart valve
Chondroitin sulfate
cartilage, bone, heart valve
heparin(sulfate)
liver, skin
Heparan sulfate
Basement membranes, components of
cell surfaces
Keratan sulfate
cornea, bone, cartilage
49
Hyaluronic acid
COO
O
Main GAG of
extracelular matrix,
not sulfated
HO
OH
4
O Ob
O
HO
1
GlcUA
3
COO
OH
NHCOCH3
O b
OH
1
GlcNAc
O
OH
50
Structure of proteoglycans
• complexes of glycosaminoglycans and specific
proteins
• the content of heteroglycans up to 95 %, chains of
10-100 monosacharide units
chains of GAG
• chains of glycosaminoglycans are covalently
attached to core protein
• most often O-glycosidic bond between protein and
vazba mezi proteinem a glycan, the terminal sequence
is Gal-Gal-Xyl
• proteoglycans can non-covalently bind hyaluronic
acid through a globular domain at the N-end of core
protein
Core protein
N-end
51
Proteoglycan aggregates
A large number of simple monomeric proteoglycans bind their
globular domains of core proteins non-covalently to a long chain of
hyaluronic acid, each of these interactions is supported by two
molecules of link glycoproteins  formation of huge aggregates esp.
in hyaline cartilages.
C-end of the
core protein of
proteoglycan
link
glycoprotein
hyaluronate
52
Various types of proteoglycans
Agrecan – tha main proteoglycan in cartilage
Versican – in many tissues, mainly vessels and skin
Dekorin – small proteoglycan of many tissues
Biglycan – small proteoglycan of cartilage
53
Fibronectin
•
•
•
•
•
•
•
•
Cellular glue (cellular adhesive)
Most abundant multiadhesive glycoprotein in connective tissue
released by fibrocytes, endothelial cells, and other cell types
the liver cells secrete fibronectin into the blood plasma (concentration
150 - 700 mg / l).
large protein  5000 AK
mediates the communication between collagen, proteoglycans and
cell surface
it interacts with receptors on the cell surface
- in this way enables the adhesion of cells to the extracellular
matrix
- enables interaction between the exterior and the interior of the
cell
it is involved in migration of the cells
54
Structure of fibronectin
• Two polypeptide chains connected near the carboxy ends
by disulfidic bond.
• Different parts of fibronectin bind to cell surface receptors,
proteoglycans, collagen and fibrin
NH3+
fibrin
heparin
collagen
NH3+
integrins
Cell membranes
(RGD receptory)
-ooc
COO-
55
Integrins
• Receptors for fibronectin
• Transmembrane proteins
• Provide a link between the
internal cytoskeleton of
cells and extracellular
proteins such as
fibronectin, collagen,
laminin.
• Are involved in a wide
variety of cell signaling
option
56
Meisenberg, Simmons:Principles of Medical Biochemistry
Main components of extracelullar matrix
Cell cytoskeleton
receptor
Fibronectine
Colagen
fibrils
Elastic fibers
Hyaluronic
acid
Meisenberg, Simmons:Principles of Medical Biochemistry
proteoglycanes
57
Composition of basement
membrane
•Basement membrane is a thin translucent sheet of
extracellular matrix with a thickness of 60 to 100 nm
•It consist of:
•
a basal lamina facing the cell
•
reticular lamina facing the extracellular matrix
•Components of basement membrane: collagen IV,
laminin, heparansulfate proteoglycans
58
Composition of basement
membrane
collagen IV
laminin
Proteoglykans
(perlecan)
nidogen
Plazmatic
membrane
integrin
Actin fibres
Meisenberg, Simmons:Principles of Medical Biochemistry
59
Laminin
• In addition to collagen IV is the main protein of basement membranes.
• Large heterotrimer (Mr 950 000)
• High affinity to ither components of basement membranes (e.g.
collagen, proteoglycans, sulfatated lipids )
• Laminin aggregates reversibly into a flat polymeric network at high
concentrations of Ca2+ ions. Those polymers bind through another
protein (either nidogen or entactin) type IV collagen and core proteins
of proteoglycans. These interactions are the cause of anchoring
epithelial cells onto components of basement membranes.
These interactions are the cause of anchoring epithelial cells onto
components of basement membranes.
60
Structure of laminin
•
•
•
•
Heterotrimer cross-shaped protein.
Polypeptide subunits , b a  are joined through disulfide bridges
Main secondary structure of all three subunits is -helix
All three chains have extension at the amino-terminal end with
globular domains.These domains allow binding of laminin to other
components of extracellular matrix.
Bindingsites for
integrine or
nidogen / entactin
clustering
of laminin
(interaction with collagen or cellular surface
heparin
61
The bone
The bone is a tissue exhibiting very high metabolic activity
The unremitting bone remodelation, both osteoresorption and bone formation,
continues after the growth of bones is finished.
The bulkiness and consistency of bones depends on the balance between
resorption and formation.
Composition of bones
Water
25 % in compact bone (12 % cementum, 10 % dentin,
1 % tooth enamel)
Organic components 30 %
Mineral components 40 %
In dry bone tissue organic components 40 – 45 %
mineral components 60 – 55 %
62
Components of extracellular matrix:
organic components - collagen type I
proteoglycans
sialoprotein
osteocalcin
citrate
mineral components - hydroxylapatite
3Ca3(PO4)2.Ca(OH)2
with octacalcium phosphate Ca8(HPO4)8.5H2O
amorphous calcium phosphate Ca3(PO4)2
on the whole 85 %
CaCO3
10 %
CaF2
0.3 %
CaCl2
0.2 %
Mg3(PO4)2
1%
alkaline salts
2%
63
Bone cells
osteoclasts (modified macrophages, bone resorption)
osteoblasts (a type of fibroblasts, bone formation)
osteocytes (transformed aged osteoblasts) affect the
activity of previous cell types by paracrine secretion of
interleukins, tumor necrosis factor (TNF), osteoprotegerin,
prostaglandins and other various growth factors.
64
Bone cells
Osteoblasts
• occur on the surfaces of growing or remodelated
bones, less frequently inside adult bones.
• they synthesize and insert osteoid (extracellular
matrix), deposit the bone minerals, and consecutively
settle and transform into osteocytes.
• the surface of osteoblasts binds molecules of alkaline
phosphate that support mineralization of the matrix.
65
Osteoclasts
• occur at sites of active bone resorption, in resorption pits.
• include many lysosomes exhibit high activity of acid
phosphatase
• osteoclasts liquidate fragments of collagen and other
organic components through phagocytosis.
66
Osteocytes
• prevailing cellular elements of mature bone
• they are dispersed in lacunas and form a cellular
net by means of contacts between their projections
• life span of osteocytes is estimated to about 25
years, extinct cells initiated bone resorption.
67
Control of bone remodelation
Bone resorption
stimulated by
parathyrin
calcitriol
inhibited by
calcitonin
estrogens
Bone formation
stimulated by (parathyrin)
calcitriol
inhibited by
androgens
estrogens
glucocorticoids
68
Biochemical markers of bone metabolism
are usefull in clinical biochemistry in tests of osteoporosis and other
metabolic osteopathies.
Biomarkers of bone formation
Catalytic concentration of the bone isoenzyme of alkaline phosphatase
(ALP) in serum – the isoenzyme is a marker of osteoblast activity,
of which it is an ectoenzyme. Total activity of ALP can serve only as a
very rough estimate due to prevalence of other isoenzymes.
Concentration of osteocalcin (bone Gla protein, BGP) in serum –
osteocalcin is a modulator of bone remodelation secreted by
osteoblasts, its production is induced by calcitriol. Osteocalcin is the
main non-collagen protein in the extracellular matrix of bones.
Concentration of N-terminal or C-terminal propeptides of procollagen I
in serum.
procollagen I N-terminal
propeptide (PINP)
tropocollagen
procollagen I C-terminal
propeptide (PICP)
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Biomarkers of bone resorption
Catalytic concentration of the bone isoenzyme of acid phosphatase
(ACP) in serum. This isoenzyme is one of the six ACP isoenzymes (called
also tartrate-resistant ACP) considered a marker of osteoclast activity.
Concentration of C-terminal telopeptide of collagen I (ICTP, or of
C-terminal octapeptide, CTx) in serum or urine. C-terminal telopeptides
are terminal non-helical sequences of tropocollagen I, without crosslinks.
Excretion of N-terminal telopeptide of collagen I (INTP, NTx) into the
urine – N-terminal non-helical sequences of tropocollagen I, which may
include some crosslinks, too..
Determination of deoxypyridinoline and pyridinoline in the urine, and
determination of galactosyl hydroxylysine in the urine.
collagen I N-terminal
telopeptide (INTP)
galactosyl-hydroxylysine
(after alkaline hydrolysis)
collagen I C-terminal
telopeptides (ICTP)
Excretion of hydroxyproline (free or total) in the urine was in common use for years,
but currently it is taken as an obsolete test of bone resorption.
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