Identification of the melatonin-binding site MT3 as the quinone reductase 2

The regulation of the circadian rhythm is relayed from the central nervous system to the periphery by melatonin, a hormone synthesized at night in the pineal gland. Besides two melatonin G-coupled receptors, mt(1) and MT(2), the existence of a novel putative melatonin receptor, MT(3), was hypothesized from the observation of a binding site in both central and peripheral hamster tissues with an original binding profile and a very rapid kinetics of ligand exchange compared with mt(1) and MT(2). In this report, we present the purification of MT(3) from Syrian hamster kidney and its identification as the hamster homologue of the human quinone reductase 2 (QR(2), EC ). Our purification strategy included the use of an affinity chromatography step which was crucial in purifying MT(3) to homogeneity. The protein was sequenced by tandem mass spectrometry and shown to align with 95% identity with human QR(2). After transfection of CHO-K1 cells with the human QR(2) gene, not only did the QR(2) enzymatic activity appear, but also the melatonin-binding sites with MT(3) characteristics, both being below the limit of detection in the native cells. We further confronted inhibition data from MT(3) binding and QR(2) enzymatic activity obtained from samples of Syrian hamster kidney or QR(2)-overexpressing Chinese hamster ovary cells, and observed an overall good correlation of the data. In summary, our results provide the identification of the melatonin-binding site MT(3) as the quinone reductase QR(2) and open perspectives as to the function of this enzyme, known so far mainly for its detoxifying properties.