ENZYMES OF ENERGY-METABOLISM IN THE BRADYZOITES AND TACHYZOITES OF TOXOPLASMA-GONDII

The bradyzoite and tachyzoite forms of Toxoplasma gondii, purified fro m infected animals, were analysed for their activities of phosphofruct okinase, pyruvate kinase, lactate dehydrogenase, NAD(+)- and NADH-link ed isocitrate dehydrogenases, and succinic dehydrogenase. Both develop mental stages contained high activities of phosphofructokinase (specif ic for pyrophosphate rather than ATP), pyruvate kinase and lactate deh ydrogenase, suggesting that energy metabolism in both forms may centre around a high glycolytic flux linked to lactate production. The marke dly higher activity of the latter two enzymes in bradyzoites suggests that lactate production is particularly important in this developmenta l form. NAD(+)-specific isocitrate dehydrogenase was not detectable in either stage of the parasite (and proved useful as a measure of the p urity of the bradyzoite preparation), whereas both parasite forms cont ained low activities of NADP(+)-linked isocitrate dehydrogenase. The r esults are consistent with the bradyzoites lacking a functional TCA cy cle and respiratory chain and are suggestive of a lack of susceptibili ty of this developmental stage to atovaquone.