Single
proteins, when analyzed with 2-D-PAGE, often show multiple spots due to PTMs. In
gels of human body fluids, the spot patterns facilitate the assignment and identification of the
proteins. We analyzed serums from patients with
congenital disorders of glycosylation (CDG) in which
glycoproteins are strongly impacted and exhibit highly distinguishable spot patterns compared to healthy controls. We detected a typical
protein pattern for alpha1-acid
glycoprotein (AGP) and
transferrin (Trf) that are markers for CDG. AGP contains five glycosylation sites which results in a complex microheterogeneity of the
glycoprotein. On the other hand, in Trf, a
glycoprotein with only two glycosylation sites, mainly biantennary complex-type-N-linked
glycans are bound. We used 2-D-PAGE, MALDI-TOF-MS, and ESI-MS for the analysis of these
glycoproteins and their corresponding
glycans. In AGP, the heterogenic glycosylation of the different glycosylation sites is responsible for the complex spot pattern. In contrast to AGP, the
protein spots of Trf cannot be explained by glycosylation. We found strong evidence that oxidation of
cysteine is responsible for the spot pattern. This study contradicts the commonly accepted assumption that the multiple
protein spots of Trf observed in 2-D-PAGE are due, as in AGP, to the glycosylation of the
protein.