Abstract

Quinine has been known to impair the functions of polymorphonuclear neutrophils (PMN). Myeloperoxidase is the most abundant enzyme found in the neutrophil. The enzyme is known to catalyse the formation of hypochloride from hydrogen peroxide and chloride, as part of the normal cellular defence system in the polymorphonuclear neutrophil. This study was carried out to investigate the effect of quinine on myeloperoxidase. To test the effects of quinine on the myeloperoxidase activity, the enzyme was isolated and purified from human blood. The effect of different parameters including concentration, pH, partition ratio estimation and kinetics of inhibition of the drug on the enzyme were then studied. The activity of the enzyme was most inhibited at 0.02 mM with partition ratio estimation of 40 enzymatic turnover cycles for complete inhibition. The inhibition of the enzyme by quinine was favoured in an acidic medium with an increase in the Michaelis Menten constant (Km) value from 0.46 to 1.0 mM and an inhibition constant Ki of 0.09 mM. It is considered that the inhibition of myeloperoxidase as demonstrated in this study by quinine may partly explain the impairment of PMN and consequent reduction of the strength of the host defense system against secondary infections.

Key words: Myeloperoxidase, quinine, inhibition, neutrophil.