Measuring the NQO2: Melatonin Complex by Native Nano-Electrospray Ionization Mass Spectrometry

Melatonin exerts its effects through a series of target proteins/receptors and enzymes. Its antioxidant capacity might be due to its capacity to inhibit a quinone reductase (NQO2) at high concentration (50 μM). Demonstrating the existence of a complex between a compound and a protein is often not easy. It requires either that the compound is an inhibitor-and the complex translates by an inhibition of the catalytic activity-or the compound is radiolabeled-and the complex translates in standard binding approaches, such as in receptology. Outside these two cases, the detection of the protein:small molecule complexes by mass spectrometry has recently been made possible, thanks to the development of so-called native mass spectrometry. Using this approach, one can measure masses corresponding to an intact noncovalent complex between a compound and its target, usually after titration or competition experiments. In the present chapter, we detail the characterization of NQO2:melatonin interaction using native mass spectrometry.