We have studied the activity of a calcium dependent transglutaminase (EC 188.8.131.52)
during the growth of the parasite Plasmodium falciparum inside the infected human
erythrocyte. There is only one detectable transglutaminase in the two-cell-system, and its
origin is erythrocytic. No activity was detected in preparations of the parasite devoid of
erythrocyte cytoplasm. The Michaelis Menten constants (Km) of the enzyme for the substrates
N'N'dimethylcaseine and putrescine were undistinguishable whether the cell extracts used in
their determination were obtained from normal or from infected red cells. The total activity of
transglutaminase in stringently synchronized cultures, measured at 0.5mM
Ca2+, decreased with the maturation of the parasite. However, a fraction
which became irreversibly activated and independent of calcium concentration was detected.
The proportion of this fraction grew with maturation; it represented only 20% of the activity in
20 hr-old-trophozoites while in 48-hr-schizonts it was more than 85% of the total activity. The
activation of this fraction of transglutaminase did not depend on an increase in the erythrocyte
cytoplasmic calcium, since most of the calcium was shown to be located in the parasite.