After different treatments with MDA-MB-231 cells, cell lysates and membrane residues were separated and then extracted with 1.2 ml of a mixture of methanol and ddH2O (5:1). Standard samples of N-pepABS with concentrations of 1, 2, 5, 10, 20, and 50 μM were prepared to gain calibration curve. Each sample was analyzed by LC20AT UHPLC (Shimadzu Corporation, Japan), with gradient elution of H2O and acetonitrile (0.1% TFA) as a mobile phase. The elution was programmed as follows: 0 to 20 min, 60 to 100%; flow rate was 1 ml/min, and the injection volume was 10 μl.