Studies of erythrocyte membrane Ca(2+)-ATPase in malaria infection

Abstract

Ca(2+)-ATPase is responsible for moving calcium out through the membrane to keep intracellular calcium at low concentration. During malaria infection, there was an increase of calcium ion level as well as an enhanced uptake of the ion in the infected red cell. Some changes on the membrane component were also reported, and it was suspected that the enzyme on mouse erythrocyte membrane might be altered or defective upon infection with P. berghei. This investigation showed that kinetic parameters of Ca(2+)-ATPase of infected membrane and parasite pellet were different from the normal membrane. There was a slight difference between the normal and infected membrane enzyme in term of pH optimum. Ca(2+)-ATPase of normal and infected membrane were almost identical in response to activator and inhibitors and quite different from the parasite pellet. Iron-porphyrin containing compounds were found to exert specific inhibitory effect on Ca(2+)-ATPase. The enzyme was very sensitive to inhibition by these compounds while (Na(+)+K(+)ATPase was found insensitive. The interaction of iron-porphyrin compounds with membrane suggested that they seemed not to involve the active site of the enzyme. The inhibition activity might be at the calmodulin binding site on the enzyme, or the compounds were acting as calmodulin antagonists. The inhibitory effect of these compounds on the enzyme was unrelated to Ca(2+) chelating effect. They caused change on both K(,m) and V(max) values of the enzyme. The inhibitors seemed to have a strong binding affinity to the membrane. Moreover, the presence of specific Ca(2+)-ATPase inhibitor in the infected lysate was also found. Its characteristic was quite similar to those compounds. This included the maximum absorption peak at 400 nm which is the characteristic of iron-porphyrin compounds. It was shown to be the factor in the infected lysate responsible for Ca(2+)-ATPase inhibition. This would give a clearer understanding of the interaction between the parasite and the host red cell membrane as well as the changes in Ca(2+) transport upon malaria infection