Lipids
Lipids play a certain role in mineralization, and mineralization
inducing molecules are very important in
atherosclerosis.[53,182,183] Atherosclerosis is an
accumulation of intimal plaques that appear in the intima of the
arteries, and the plaques consist mainly of lipoproteins, oxidized
lipoproteins, various cellular debris, and finally, groups of apatite
deposits.[135,184] The apatite formed by
calcification of the medial arteries of the arterial intima is embedded
in the lipid deposits.[126] There are individual
and regional differences in atherosclerosis, which makes it impossible
to understand the pathogenesis.
Other proteins
A portion of the vertebrate body is biomineralized by proteins other
than collagen fibers. The organic matter in the enamel consists of
AMBN,
AMELX, ENAM, MMP20, ALP, and KLK4.
When AMELX is secreted by ameloblasts to the outside of cells, it exists
in the form of a monomer or nano
aggregate.[43,44,185,186] The inside of nano
aggregate is a hydrophobic core, and the outside is a hydrophilic
retractable fragment.[187–190] In dependence on
temperature, pH, and protein concentration, AMELX self-assembles to form
a linear morphology that provides nucleation sites for hydroxyapatite
(Figure 5 ).[191–193] This linear
morphology may be the key to controlling the directional growth of
hydroxyapatite.[194–196] AMBN and ENAM are all
present in enamel nucleation and crystallization, but their specific
roles are not yet clear.
Interestingly, AMBN, AMELX, and ENAM were not found in the mature
enamel. It was found that the expression and secretion of MMP20 and KLK4
are spatiotemporal throughout the process of enamel
mineralization.[28,38,197] MMP20 and KLK4 induce
the degradation of enamel matrix proteins, providing space for
hydroxyapatite crystal growth and making enamel
harder.[40,197] When Raman spectroscopy is used to
analyze enamel, the Amide I, II, and III bond is more of a
characteristic peak of enamel matrix
protein.[58,198–200] A typical example is the
markedly enhanced intensity of the Amide III in enamel after MMP20
knockdown, indicating that the absence of MMP20 expression results in
enamel matrix proteins being blocked in the
enamel.[145]
Other minerals
Calcium oxalate stones are formed under pathological conditions. Its
chemical formula is CaC2O4 or Ca
(COO)2, fundamentally different from hydroxyapatite and
carbonate.[137,201–203] The Raman characteristic
peaks of calcium oxalate are also distinct. Characteristic bands for
calcium oxalate monohydrate are assigned to the C-O asymmetric stretch
(1630 cm–1), C-O symmetric stretch (1485 and 1460
cm–1), and C-C stretch (895 cm–1),
and O-C-O in-plane bending (500
cm–1).[139,204–206] The
pathological environment is more complex, and the embedding of different
organic matter also causes changes in the Raman spectroscopy of
biomineralization. The exact trend of the changes is not clear. In
addition to calcium oxalate stones, there are xanthine stones, cystine
stones, ammonium magnesium phosphate stones, uric acid (urate) stones,
and calcium phosphate stones.[207–209]