Is There Sufficient Evidence that the Melatonin Binding Site MT3 Is Quinone Reductase 2?

In the 1980s, researchers used binding studies to show that there is a melatonin binding site in addition to the receptors described previously. It was first termed ML2 and then, in 1999, MT3. Purification efforts led to its identification as quinone reductase 2. Several lines of evidence support the notion that MT3 is the same as quinone reductase 2, including the detection and characterization of MT3 whenever quinone reductase 2 was added to various systems under various conditions. This evidence is discussed in this review, which summarizes the results of relevant cellular and animal experiments. The recent discovery that the quinone reductase 2 enzyme can be partly membrane-associated may unite the current body of evidence and allow us to conclude definitively that the third melatonin binding site, MT3, is indeed quinone reductase 2.