Phosphatidylinositol Synthase of Tetrahymena: Inositol Isomers as Substrates in Phosphatidylinositol Biosynthesis and Headgroup Exchange Reactions

Abstract

Phosphatidylinositol (PtdIns) synthase in microsomal fractions derived from Tetrahymena vorax was studied to determine its activity requirements. The suitability of inositol isomers as substrates for the synthase and in headgroup exchange reactions also was investigated. Tetrahymena PtdIn synthase activity was optimum in the presence of 2 mM MgCl2 plus 2 mM MnCl2, a pH of 7.8, and a temperature of 30 degrees C. The enzyme retained approximately 80% of its activity after incubation at 70 degrees C for 10 min. PtdIns headgroup exchange activity was maximal in the presence of cytidine monophosphate. By following either the accumulation of radiolabeled reaction products or the loss of radiolabel from precursors, each of the inositol isomers tested appeared to serve as substrates for both the PtdIns synthase and PtdIns:inositol phosphatidyl transferase activities. In each case, myo-inositol and scyllo-inositol were the preferred substrates. The data suggest two routes for the formation of phosphatidyl-non-myo-inositols in Tetrahymena and the potential for the production of novel, non-myo-inositol-containing second messengers.

Conservation and Innovation in Tetrahymena Membrane Traffic: Proteins, Lipids, and Compartments

“…This reaction has been characterized in T. vorax microsomes, where the substrates included both myo - and non- myo -inositol isomers(Riggs et al, 2007). T. thermophila has a single PIS gene (TTHERM_00678350) whose expression profile is similar to the enzymes involved in the synthesis of inositol (MIPS and InsPase).…”

Identification of two scyllo-inositol dehydrogenases in Bacillus subtilis

“…scyllo-Inositol (SI), the C-2 epimer of MI, is relatively rare but is sometimes found in plants, for example in barley seeds (Kinnard et al, 1995) and chrysanthemum buds (Ichimura et al, 2000). Also, SI is sometimes present in phosphatidylinositol, since the phosphatidylinositol synthase of Tetrahymena vorax is known to act not only on MI but also on SI (Riggs et al, 2007). In mammals, SI has been shown to localize in the kidney and brain (Sherman et al, 1968), and an enzyme from bovine brain was found to interconvert MI and SI (Hipps et al, 1976).…”