hemoglobin glycation as a nonenzymatic reaction caused by high-glucose concen-
tration in blood is validated and frequently used as diagnostic biomarker for
diabetes mellitus.^70
Together with phosphorylation, glycosylation is one of most frequent (and most
frequently investigated) protein PTMs. Changes of this posttranslational modifi-
cation can significantly influence protein function and cellular localization. These
changes are frequently the consequence of pathological processes in organisms.
Consequently, some types of protein glycosylation are frequently discussed as
potential disease, teranostic, and cellular biomarkers.^71 The term “glycome” that
was coined about 10–15 years ago means the complete content of glycoproteins in
an organism. There are many possibilities for glycosylation of one particular
protein. The logical result is that the glycome is much more complex than the
proteome. First glycomic investigations were focused on the analysis of complete
glycans in a biological sample after their cleavage from protein parts. As a result, an
average glycan profile, e.g. in serum, urine or tissue sample, can be obtained.
Statistical analysis can finally define differences in glycosylation between control
and patient samples.^72 Development of high-throughput methods for analysis
enables fast glycan profiling, and subsequent statistical analysis yields in the
determination of specific structures as biomarker candidates. However, a biological
sample can contain thousands of glycoproteins, and this method approach gives
only a superficial look into this kind of PTMs. It is the reason that changes in
glycosylation of specific proteins that are isolated from biological samples are
recently in the focus of this kind of analyses. First step in the analysis of a particular
protein glycosylation is its isolation from a complex biological mixture, possibly by
use of one of high-throughput methods. IgG is one of the first proteins that were
analyzed, and change of its glycosylation during aging and also during pathological
processes could be defined.^73 Changes of IgG glycosyaltion in cancer and in some
chronical inflammatory diseases could be found, and some glycan types are
discussed as potential biomarker candidates.^74 Interestingly, changes in glycosyl-
ation of another plasma protein, transferrin, are used for a long time as a biomarker
for chronic alcoholism, and their exact determination by use of MS has been also
developed.^75 Recently, methods for high-throughput analysis of other antibodies
such as IgA, IgE, and IgM were developed. They shall be applied for the determi-
nation of changes of their glycosylation as a result of pathological changes in the
organism (Plomp et al. 2014 ). However, the development of high-throughput
methods for the isolation of these proteins before cleavage and analysis of their
glycans is necessary, and some chromatographic analyses that enable the
(^70) Silvestri et al. ( 2013 ), pp. 309–316.
(^71) Christiansen et al. ( 2014 ), pp. 525–546.
(^72) Pivac et al. ( 2011 ).
(^73) Pucic et al. ( 2011 ).
(^74) Huhn et al. ( 2009 ), pp. 882–913.
(^75) Del Castillo Busto et al. ( 2009 ), pp. 1009–1113.
192 D. Josic ́and U. Andjelkovic ́