Biology Reference
In-Depth Information
METABOLOMICS FOR BIOMARKER
DISCOVERY
the wide range of concentrations of the com-
pounds (from pmol to mmol).
The study of metabolites was for a long time
limited to the analysis of a restricted number of
molecules, but thanks to recent analytical devel-
opments, it has now evolved to enable the detec-
tion of several hundred to several thousand
analytes in a single sample. Metabolomics has
proven to be a powerful tool to assess the chem-
ical diversity of biological systems in a holistic
manner. 1 This approach aims at the quantitative
unbiased description of the overall molecular
events characterizing living systems. The infor-
mation provided by the comprehensive pro
MASS SPECTROMETRY-BASED
METABOLOMICS
Several analytical platforms are available for
the untargeted monitoring of the metabolic
content of biological samples but two tech-
niques constitute prominent tools applied in
the vast majority of metabolomic studies,
namely nuclear magnetic resonance (NMR) and
mass spectrometry (MS). Although early metab-
olomic studies relied mainly on NMR, MS has
emerged as a powerful and complementary
approach thanks to its capacity to monitor
multiple components with high sensitivity. Clas-
sically, two strategies are available, namely
direct MS methods and hyphenated methods
involving separation techniques and MS such
as gas chromatography (GC-MS), liquid chroma-
tography (LC-MS), and capillary electrophoresis
(CE-MS). 4
ling
of low molecular weight molecules is of primary
importance for monitoring and understanding
the biological processes of an organism, an
organ, a speci
uid under phys-
iological conditions or its dynamic response to
pathological, environmental, or developmental
perturbations. As it is able to characterize the
biochemical phenotype of the system under
study, metabolomics can be considered as the
actual, most functional of the
c tissue, or a bio
disci-
plines. 2 Metabolic pathways establish the canvas
for the functioning of cellular components. As
a consequence, it constitutes a pivotal point in
systems biology. Metabolites are the down-
stream products of cellular regulatory processes
and therefore constitute excellent potential
biomarker candidates for many pathological or
clinical situations. The high sensitivity of metab-
olite levels to biological variations constitutes
another important advantage and the discovery
of relevant biomarkers is expected to help to
understand the relationships between pathways
and biochemical processes.
On the other hand, the great variety of these
small compounds in terms of physicochemical
properties such as molecular weight, polarity,
and solubility is a critical aspect to consider for
their analysis. 3 Additionally, the experimental
procedure is expected to handle appropriately
omics
Direct MS Methods
Direct injection mass spectrometry (DIMS) is
the method of
choice
to provide high-
throughput
fingerprints and is often used for
rapid sample classi
cation. As all compounds
within a sample are injected simultaneously in
the MS device, high-resolution analyzers are
preferred because of their higher separative
power. DIMS is however of limited use for
compound identi
cation and additional dedi-
cated experiments are often required. Fragmen-
tation patterns can be obtained with tandem
MS to provide additional
information for
compounds identi
cation. DIMS experiments
are usually carried out with atmospheric pres-
sure ionization, such as electrospray ionization
(ESI) or atmospheric pressure chemical ioniza-
tion (APCI). Signal suppression due to matrix
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