Fig. 7.

Determination of location of second O-glycosylation site. Nano-LC MS spectra of the reaction products of apoE tryptic peptides after treatment with methylamine are shown. a, MS/MS of [M + 2H]²⁺ ion (m/z 810.85) of apoE(283–299) (inset b). c, MS/MS of the [M + 2H]²⁺ ion (m/z 817.35) of apoE(283–299) modified by methylamine after treatment with CH₃NH₂ and β-elimination of the O-glycan (inset d). e, sequence-specific b- and y-type ions identified in both spectra. Intense product ions indicate conversion of Ser²⁹⁰ to Ser²⁹⁰-methylamine, indicating that the O-glycosylation site was predominately on Ser²⁹⁰ (product ions containing Ser²⁹⁰ were shifted by +m/z 13). Spectra were recorded using a Q-Tof Ultima with precursor/production ion resolution ∼10,000 and a root mean square error of ∼50 ppm. Rel Abun, relative abundance.