Norepinephrine, a hormone and a neurotransmitter, was known as catecholamine. In the present study, we examined the inhibitory effect of norepinephrine on lactoperoxidase enzyme purified from bovine milk. Lactoperoxidase (LPO; E.C.126.96.36.199) was purified from bovine milk with three purification steps: Amberlite CG-50 resin, CM-Sephadex C-50 ion-exchange chromatography and Sephadex G-100 gel filtration chromatography, respectively. LPO was purified with a yield of 31.5%, a specific activity of 30.33 EU/mg proteins and 20.77 purification fold. To determine enzyme purity, SDS-PAGE was performed and single band was observed. The R-z (A(412)/A(280)) value for LPO was 0.9. The effect of norepinephrine on lactoperoxidase was determined using ABTS as a chromogenic substrate. The half maximal inhibitory concentration (IC50) value norepinephrine was found to be 67.2 mu M. Also, inhibition constant (K-i) for norepinephrine was found to be 62.0 mu M. Norepinephrine was found as noncompetitive inhibitor.