Melatonin is the chief secretory product of the pineal gland and is synthesized enzymatically from serotonin. These indoleamine derivatives play an important role in the prevention of oxidative damage. Lactoperoxidase (LPO; EC 220.127.116.11) was purified from bovine milk with three purification steps: Amberlite CG-50 resin, CM-Sephadex C-50 ion-exchange, and Sephadex G-100 gel filtration chromatography, respectively. LPO was purified with a yield of 21.6%, a specific activity of 34.0 EU/mg protein, and 14.7-fold purification. To determine the enzyme purity, SDS-PAGE was performed and a single band was observed. The R-z (A(412)/A(280)) value for LPO was 0.9. The effect of melatonin and serotonin on lactoperoxidase was determined using ABTS as chromogenic substrate. The half-maximal inhibitory concentration (IC50) values for melatonin and serotonin were found to be 1.46 and 1.29 mu M, respectively. Also, the inhibition constants (K-i) for melatonin and serotonin were 0.82 +/- 0.28 and 0.26 +/- 0.04 mu M, respectively. Both melatonin and serotonin were found to be competitive inhibitors.