Kinetic investigation of myeloperoxidase upon interaction with copper, cadmium, and lead ions

Myeloperoxidase (MPO), which is abundantly expressed in neutrophils, catalyzes the formation of a number of reactive oxidant species. However, evidence has emerged that MPO-derived oxidants contribute to tissue damage and initiation and propagation of inflammatory diseases, particularly, cardiovascular diseases. Therefore, studying the regulatory mechanisms of the enzyme activity is of great importance. For clarifying some possible mechanism of the enzyme activity, kinetic investigations of MPO in the presence of Copper (Cu), Cadmium (Cd), and Lead (Pb) ions were carried out in vitro. MPO was partially purified from human white blood cells using ion-exchange and gel-filtration chromatography techniques. Its activity was measured spectrophotometrically by using tetramethyl benzidine (TMB) as substrate. Purified enzyme had a specific activity of 21.7 U/mg protein with a purity index of about 0.71. Cu inhibited MPO activity progressively up to a concentration of 60 mM at which about 80% of inhibition achieved. The inhibition was non-competitive with respect to TMB. An inhibitory constant (Ki) of about 19 mM was calculated from the slope of repot. Cd and Pb did not show any significant inhibitory effect on the enzyme activity. The results of the present study may indicate that there are some places on the enzyme and enzyme-substrate complex for Cu ions. Binding of Cu ions to these places result in conformational changes of the enzyme and thus, enzyme inhibition. This inhibitory effect of Cu on the enzyme activity might be considered as a regulatory mechanism on MPO activity.