RELATIVE IMPORTANCE OF INOSITOL (1,4,5)TRISPHOSPHATE AND INOSITOL (1,3,4,5)TETRAKISPHOSPHATE IN ENTAMOEBA-HISTOLYTICA

[H-3]Inositol tetrakisphosphate (Ins(1,3,4,5)P-4) binding sites which were poorly displaced by unlabelled inositol (1,4,5)-trisphosphate (In s(1,4,5)P-3) were detected in membrane fractions of Entamoeba histolyt ica. Similarly, unlabelled Ins(1,3,4,5)P-4 was 30-fold less efficient in displacing [H-3]Ins(1,4,5)P-3 binding, pH sensitivities of binding of the two isomers were markedly different, Scatchard analysis of the data revealed single binding sites and similar receptor densities for each of the two isomers, Formation of both Ins(1,4,5)P-3 and Ins(1,3,4 ,5)P-4 in E. histolytica was also demonstrated, Calcium release studie s showed that after treatment with a saturating dose of either Ins(1,4 ,5)P-3 or Ins(1,3,4,5)P-4 the other inositol polyphosphate could parti ally revive the response to a subsequent addition of the first inducer , Our data clearly demonstrate that Ins(1,4,5)P-3 and Ins(1,3,4,5)P-4 are two equally important but independent second messengers in E. hist olytica.