Melatonin does not “directly scavenge hydrogen peroxide”: demise of another myth

We have reexamined claims that melatonin directly scavenges hydrogen peroxide and shown them to be unfounded. Relative hydrogen peroxide concentrations were determined in the absence and presence of melatonin using both an isoluminol-based chemiluminescence assay (with measurements at circa 40 s, 6 h, and 24 h after mixing) and the phenol red/horseradish peroxidase assay employed by two earlier groups of workers (with measurements at 5 s, then every minute for the first 5 min, and then every hour to 5 h). Both assay procedures were in agreement. There was no significant change in the hydrogen peroxide concentrations over 24 h, and, furthermore, the concentrations of H2O2 in the presence and absence of melatonin were the same within experimental error. Our results were obtained in metal ion-free systems. It therefore appears likely that the claims for a direct melatonin/H2O2 reaction were due to contamination by traces of transition metal ions.