Fig 2.

Direct binding of zafirlukast to Lsr2 using intensity-fading MALDI mass spectrometry. (A) Shown are the MALDI-TOF spectra of different mixtures of zafirlukast, ritonavir, and Lsr2. The concentration of Lsr2 in each drop increases from top to bottom, promoting a decrease in the relative intensity of the zafirlukast-plus-Na⁺ ion peak (m/z 598.5) relative to the ritonavir-plus-Na⁺ ion peak (m/z 743.9). (B) Plot of the ion ratio of zafirlukast (Z) over ritonavir (R) versus the concentration of Lsr2. As the concentration of Lsr2 is increased, a larger portion of the zafirlukast is bound to the protein, thereby suppressing the amount of the free ion and decreasing the intensity of the zafirlukast signal.