INHIBITION OF PORCINE BRAIN INOSITOL 1,3,4-TRISPHOSPHATE KINASE BY INOSITOL POLYPHOSPHATES, OTHER POLYOL PHOSPHATES, POLYANIONS AND POLYCATIONS

We have partially purified an enzyme activity that phosphorylates inos itol 1,3,4-trisphosphate from porcine brain, rat liver and bovine test is by FPLC chromatography on Q-Sepharose anion-exchange resin and Hepa rin-agarose. The products of this reaction were inositol 1,3,4,6-tetra kisphosphate and inositol 1,3,4,5-tetrakisphosphate. The same enzyme a ppears to be responsible for both 6-kinase and 5-kinase activities aga inst inositol 1,3,4-trisphosphate (the 6-kinase: 5-kinase activity rat io is approximately 4 to 1), has a pH optimum of similar to 6.8 and re quires Mg2+ for activity. The K-m values of the enzyme for inositol 1, 3,4-trisphosphate and ATP were similar to 0.5 mu M and similar to 100 mu M, respectively. Inositol 3,4,5,6-tetrakisphosphate, inositol 1,3,4 ,6-tetrakisphosphate and inositol 1,3,4,5-tetrakisphosphate are all co mpetitive inhibitors with K-i values of 0.4 mu M, 3 mu M and 5 mu M, r espectively, well within their likely intracellular concentration rang es: they inhibited 6-kinase and 5-kinase activities equally. 2,3-Bisph osphoglycerate and spermine were also competitive inhibitors, with K-i values of 0.8 mM and 12 mM, respectively. Dextran sulphate was a non- competitive inhibitor with a K-i of similar to 15 mu M, and poly-L-lys ine (IC50 similar to 200 mu M), polyvinylsulphate (IC50 similar to 250 mu M) and heparin(IC50 similar to 2 mg/ml) also inhibited. Inhibition by these compounds suggests that inositol 3,4,5,6-tetrakisphosphate ( and to a lesser extent inositol 1,3,4,5-tetrakisphosphate and other na turally occurring intracellular ions) may restrict the synthesis of in ositol 1,3,4,6-tetrakisphosphate and hence regulate the rate of inosit ol penta- and hexakisphosphate synthesis from receptor-generated inosi tol phosphates.