MYOINOSITOL PENTAKISPHOSPHATES - STRUCTURE, BIOLOGICAL OCCURRENCE AND PHOSPHORYLATION TO MYOINOSITOL HEXAKISPHOSPHATE

1. Standard and high-performance anion-exchange-chromatographic techniques have been used to purify myo-[H-3]inositol pentakisphosphates from various myo-[H-3]inositol-prelabelled cells. Slime mould (Dictyostelium discoideum) contained 8-mu-M-myo-[H-3]inositol 1,3,4,5,6-pentakisphosphate, 16-mu-M-myo-[H-3]inositol 1,2,3,4,6-pentakisphosphate and 36-mu-M-D-myo-[H-3]inositol 1,2,4,5,6-pentakisphosphate [calculated intracellular concentrations; Stephens & Irvine (1990) Nature (London) 346, 580-583]; germinating mung-bean (Phaseolus aureus) seedlings contained both D- and L-myo-[H-3]inositol 1,2,4,5,6-pentakisphosphate (which was characterized by P-31 and two-dimensional proton n.m.r.) and D- and/or L-myo-[H-3]inositol 1,2,3,4,5-pentakisphosphate; HL60 cells contained myo-[H-3]inositol 1,3,4,5,6-pentakisphosphate (in a 500-fold excess over the other species), myo-[H-3]inositol 1,2,3,4,6-pentakisphosphate and D- and/or L-myo-[H-3]inositol 1,2,4,5,6-pentakisphosphate; and NG-115-401L-C3 cells contained myo-[H-3]inositol 1,3,4,5,6-pentakisphosphate (in a 100-fold excess over the other species), D- and/or L-myo-[H-3]inositol 1,2,4,5,6-pentakisphosphate, myo-[H-3]inositol 1,2,3,4,6-pentakisphosphate and D- and/or L-myo-[H-3]inositol 1,2,3,4,5-pentakisphosphate. 2. Multiple soluble ATP-dependent myo-inositol pentakisphosphate kinase activities have been detected in slime mould, rat brain and germinating mung-bean seedling homogenates. In slime-mould cytosolic fractions, the three myo-inositol pentakisphosphates that were present in intact slime moulds could be phosphorylated to myo-[H-3]inositol hexakisphosphate: the relative first-order rate constants for these reactions were, in the order listed above, 1:8:31 respectively (with first-order rate constants in the intact cell of 0.1, 0.8 and 3.1 s-1, assuming a cytosolic protein concentration of 50 mg/ml), and the K(m) values of the activities for their respective inositol phosphate substrates (in the presence of 5 mM-ATP) were 1.6-mu-M, 3.8-mu-M and 1.4-mu-M. At least two forms of myo-inositol pentakisphosphate kinase activity could be resolved from a slime-mould cytosolic fraction by both pharmacological and chromatographic criteria. Rat brain cytosol and a soluble fraction derived from germinating mung-bean seedlings could phosphorylate myo-inositol D/L-1,2,4,5,6-, D/L-1,2,3,4,5-, 1,2,3,4,6- and 1,3,4,5,6-pentakisphosphates to myo-inositol hexakisphosphate: the relative first-order rate constants were 57:27:77:1 respectively for brain cytosol (with first-order rate constants in the intact cell of 0.0041, 0.0019, 0.0056 and 0.000073 s-1 respectively, assuming a cytosolic protein concentration of 50 mg/ml) and 1:11:12:33 respectively for mung-bean cytosol (with first-order rate constants in a supernatant fraction with a protein concentration of 10 mg/ml of 0.0002, 0.0022, 0.0024 and 0.0066 s-1 respectively).