To denature proteins, 250 μL 1% formic acid in ACN were added and samples were vortexed for 3 min. The solution was added to 100 mg anhydrous MgSO4 and 25 mg NaCl and again vortexed for 3 min, followed by centrifugation at 18,000  g for 10 min at 4°C. The upper organic phase was stored at -20°C for at least 2 h to precipitate proteins and again centrifuged at 18,000  g for 10 min at 4°C. The supernatant was then lyophilized with the vacuum concentrator at temperatures between 4°C and 15°C until dry. The residue was reconstituted in 250 μL ACN/H2O (10/90, v/v), vortexed and centrifuged at 18,000  g for 10 min at 4°C. The supernatants were transferred to amber glass vials for LC-MS/MS measurements and stored at -20°C until analysis.