LC-MS/MS analyses of the labeled peptides were carried out using a Q-Exactive mass spectrometer (Thermo Fisher Scientific, San Jose, CA, USA) combined with an EASY-nLC 1200 system (Thermo Fisher Scientific). After loading 2 μg of the labeled peptides onto a C18 reversed phase HPLC column (Thermo Fisher Scientific), peptides were chromatographed for 120 min at a flow rate of 300 nL/min over gradients from 2–80% (mobile phase A comprising 0.1% (v/v) formic acid and 2% (v/v) acetonitrile; mobile phase B comprising 0.1% (v/v) formic acid and 80% (v/v) acetonitrile). The following ion source parameters, including spray voltage 1.8 kV, capillary temperature 275 °C and declustering potential 100 V, were set. The mass spectrometer was run using a data-dependent Top-20 acquisition mode, switching automatically between MS and MS/MS. A full MS scan ranging from 350 to 1300 m/z was conducted at 70 000 resolution with an automatic gain control (AGC) target value of 3 × 106 ions and a maximum ion transfer (IT) of 20 ms. The precursor ions were fragmented by means of high-energy collisional dissociation (HCD), and all MS/MS spectra were scanned using the following parameters: resolution 17 500; AGC 1 × 105 ions; maximum IT 50 ms; dynamic exclusion duration 18 s; normalized collision energy 30%; and intensity threshold 1.6 × 105.