Assessment of metabolic incorporation of azido-modified sialic acids into cellular glycans. (A) Cell surface labeling of SW1990 cells incubated with 1,3,4-O-Bu3ManNAz for two days. Labeling of the resulting surface azido-glycoconjugates, indicated by green fluorescence, was achieved by click chemistry with an Alexa488 fluorophore and the cells were counterstained with DAPI (blue fluorescence) as a nucleus marker. (B) Proteins were harvested from untreated control (denoted by “C”) and treated (denoted by “T”) SW1990 cells and the specific biotinylation of azide-modified proteins was assessed by Western blots (“M” denotes molecular weight markers). (C) Silver staining of PAGE-separated proteins from control (denoted “C”) or treated (denoted “T”) SW1990 cells verified that similar amounts of protein were analyzed in both cases.