Figure 1 - The QUIC-Tag strategy for quantitative analysis of O-GlcNAc.

O-GlcNAc–modified proteins (blue squares) from two samples are specifically labeled with a ketone-containing galactose (yellow circles), which is then further reacted with a biotin moiety (green hexagon). After tryptic digestion, light (purple) or heavy (red) methyl groups are added to the amino groups of the peptides, and the O-GlcNAc–modified peptides are enriched via avidin. Both identification (most robustly performed by ETD fragmentation) and relative quantification can then be achieved.