Melatonin binds with high affinity and specificity to beta-amyloid: LC-MS provides insight into Alzheimer's disease treatment

To study the potential relationship between melatonin and beta-amyloid (Abeta), we established a liquid chromatography-mass spectrometry (LC-MS) method to quantitatively analyze melatonin, deuterated isotopes (melatonin-D4), and internal standard 6-iodo-2-(4'-dimethylamino-) phenyl-imidazo(1,2) pyridine (IMPY) under positive (+) mode. The gradient elution was set to 6 min, and the corresponding peak time of melatonin and its isotope melatonin-D4 was 3.14 min, while the peak time for the internal standard IMPY was 3.24 min. Next, we established and optimized the molecule receptor saturation binding assay based on LC-MS to determine the melatonin affinity for beta-amyloid (Abeta). Melatonin showed a high and specific binding for Abeta. The corresponding equilibrium dissociation constant (Kd) of melatonin with Abeta 1-40 and Abeta 1-42 was 814.37 +/- 36.62 and 628.33 +/- 13.57 nmol center dot L-1; besides, the Kd of melatonin with mixed plaques (1-40 and 1-42) was 461.13 +/- 45.37 nmol center dot L-1. The results may suggest the potential mechanism of action of MT on Abeta and provide a theoretical basis for the improvement of MT treatment of Alzheimer's disease.