Superoxide dismutase (SOD), a metal containing enzyme is present in parasite Leishmania donovani as well as in host macrophages both resident and activated in a detectable amount, although the level is much higher in the latter case. It is observed that at any particular protein concentration, the SOD activity is highest in the case of parasite infected macrophages and lowest in the case of normal resident macrophages; the SOD activity of thioglycolate activated macrophages lies in between the two. It is also noticed that formalin-killed Leishmania donovani neither attach to macrophages nor do they increase the SOD activity of the host. Thus, the processes, e.g. attachment of the parasite to the host membrane, subsequent membrane perturbation and thus activation of membrane bound enzyme NADPH oxidase leading to respiratory burst, may be responsible for an enormous increase in the SOD level in macrophages during infection. Moreover, the chemical nature of the SOD found in infected macrophages has been investigated by using an inhibitor, e.g. NaCN, which specifically inhibits Cu-Zn SOD but not Fe-SOD. A considerable inhibition of SOD activity by NaCN in infected macrophages confirms the chemical nature of the increased SOD to be of Cu-Zn type, usually found in host. Presumably, Cu-Zn SOD or host SOD plays a protective role at the time of parasite infection although the role of parasitic SOD or some other mechanisms for the survival of the parasite within the toxic phagolysosome environment, of the macrophage cannot be ruled out.

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