Biomedical Engineering Reference
In-Depth Information
through hydrophobic bonds. The size of the micelles decreases as the C-terminal
and 13 kDa regions are sequentially cleaved by MMP-20 [
65
] and EMSP-1 [
66
]
(step 1 and step 2, respectively; Fig.
5.2
b). The C-terminal and 13 kDa regions are
removed from the micelles, leaving behind smaller micelles comprised of 6 kDa
fragments. This provides growth space for enamel crystal.
The conformation of amelogenin molecules in solution was determined using
circular dichroism (CD) and small-angle X-ray scattering (SAXS). These studies
revealed that amelogenin molecules, which consist of some 180 amino acid residues,
do not behave like a globular protein in solution. Porcine amelogenin molecules
(20 kDa) consist of discrete folding units in the N-terminal, central, and C-
terminal regions and have a
-sheet structure, a polyproline structure, and a random
coil structure, respectively [
67
]. In 0.1-0.3
“
g protein/ml 2% (v/v) acetic acid
solution (pH5.3, 25
ı
C), 20 kDa amelogenin forms an elongated bundle structure
and assumes a monomeric form [
67
], while in 10
g protein/mL 2% (v/v) acetic
5.3, 5
ı
C), 20 kDa amelogenin (length of 15 nm and diameter of
2nm)(Fig.
5.2
c) assumes a dimmer form and exhibits an overall radius of gyration
of 4.4 nm [
68
]. When rodlike molecules with both hydrophobic and hydrophilic
terminals aggregate, formation of a micelle structure cannot be ruled out.
acid solution (pH
<
5.2.5
Ionic Composition and pH of Enamel Fluid
Separation of the enamel fluid from the secretory stage of the matrix enabled the
ionic composition to be measured and the degree of saturation of the enamel fluid
to be calculated with respect to the enamel HAP and related calcium phosphate [
69
,
70
]. The water content in the secretory stage was around 30-40% of the wet weight
[
12
]. The average composition of the enamel fluid during the secretory stage was
total [Ca
2C
]0.5mM,total[P]3.9mM,[Mg
2C
]0.8mM,[Na
C
] 140 mM, [K
C
]
20 mM, [Cl
] 150 mM, [F
]5
10
3
mM, and total [HCO
3
]9.74mM.ThepH,
the ionic strength, and the osmolarity were 7.26, 165 mM, and 312 mosmol/kg
H
2
O, respectively. The pH was measured under a constant CO
2
partial pressure.
The concentration of free Ca
2C
ions (0.15 mM) was about 30% of the total
Ca
2C
concentration; the remaining 70% were bound with other inorganic ions and
proteins. The total concentration of Ca
2C
was about half that in the predentin
(1.0 mM) and in the fluid of enamel organ tissue surrounding the developing
teeth (1.3 mM) and was much lower than that in the blood plasma (2.5 mM).
On the other hand, the PO
4
3
concentration of the enamel fluid (3.5 mM) was
about three times that in the blood plasma (1.1 mM). The activity of Ca
2C
,(
a
Ca
),
calculated taking into account the thermodynamic factors, was 0.053 mM. The
calculated free HCO
3
concentration in enamel fluid was around 10 mM, lower
than that of the blood plasma CO
3
2
concentration (about 22 mM). The calculated
degree of supersaturation with respect to H-Ap, OCP, and enamel HAP suggests
that the enamel fluid was supersaturated with H-Ap, undersaturated with OCP,
undersaturated with enamel HAP at a pH lower than 7.1, and supersaturated with
