Of inositol phosphates

Paper chromatography of inositol phosphates obtained from soil extracts is described. ... 

The metabolism of inositol phosphates leads to regeneration of free inositol 354... 

FIGURE 20-7 Pathways of inositol phosphate metabolism. Note that the metabolism of the second messenger I(1,4,5)P3 is shown to the left of the dashed line, while the interconversions of the higher inositol phosphates are shown to the right of the dashed line. Only the quantitatively major established pathways are depicted. Li+ is known to block the dephosphorylation reactions indicated by the (black) bars. Numbers refer to the following enzymes 1, inositol polyphosphate 5-phosphatase (I) 2, inositol polyphosphate 1-phosphatase 3,I(1,4,5)P3 3-kinase 4, inositol polyphosphate 4-phosphatase 5, inositol polyphosphate 3-phosphatase 6, inositol monophosphate phosphatase 7, I(1,3,4)P3 6-kinase/I(3,4,5,6)P4 1-kinase 8, Ipmk 9, DIPP 10, IP6 kinase 11, Ipk 1 12, MIPP 13, PP-IP5 kinase. 

Activation of neutrophils with PAF occurs through a G-protein-linked receptor, and the subsequent transmembrane signalling involves the stimulation of inositol phosphate metabolism. Within 30 s of addition of PAF (0.01-100 nM), intracellular Ca2+ levels increase and Ca2+ transport from the external medium is enhanced. It seems that phospholipase C-dependent and -independent activation pathways are involved in Ca2+ mobilisation. This indirectly suggests that two receptors may be involved in PAF activation. The first of these is pertussis-toxin-insensitive and may be linked to a... 

The formation of inositol phosphates is a good measure of the activity of PLC. Within seconds of addition of fMet-Leu-Phe to neutrophils, Ins 1,4,5-P3 formation is detected, and levels of this compound increase for about 20-30 s, falling to their original levels within 2 min of stimulation. It is likely that this decrease in Ins 1,4,5-P3 occurs because it is phosphorylated to inositol 1,3,4,5-tetrakisphosphate (Ins 1,3,4,5-P4) and then subsequently dephosphorylated to inositol 1,3,4-trisphosphate (Ins 1,3,4-P3). This latter molecule can be converted into inositol 1,4-bisphosphate, then into inositol... 

Figure 6.9. Pathways of inositol phosphate metabolism. Ins 1,4,5-P3, generated via the hydrolysis of phosphatidyl 4,5-bisphosphate by phospholipase C, can be metabolised by a kinase (to generate Ins 1,3,4,5-P4) or via a phosphatase (to yield Ins 1,4-P2). These products can be metabolised further to produce inositol, which itself may be sequentially phosphory-lated to regenerate phosphatidylinositol 4,5-bisphosphate.

Metabotropic receptors, in contrast, create their effects by activating an intracellular G protein. The metabotropic receptors are monomers with seven transmembrane domains. The activated G protein, in turn, may activate an ion channel from an intracellular site. Alternately, G proteins work by activation or inhibition of enzymes that produce intracellular messengers. For example, activation of adenylate cyclase increases production of cyclic adenosine monophosphate (cAMP). Other effector mechanisms include activation of phospholipases, diacylglycerol, creation of inositol phosphates, and production of arachidonic acid products. Ultimately, these cascades can result in protein phosphorylation. 

Kozawa et al, 2000a and b). In both cell types, the p38 MAPK-specific inhibitor, SB203580, blocks SIP hsp27 induction and amplification of inositol phosphates, hi osteoblastic cells, PGE shmulates cyclic AMP formation and inhibits apoptosis. This appears to be mediated by a cyclic AMP-dependent modulation of SPHK and S IP production (Machwate etal., 1998)... 

Note that equation 5.2 is irreversible and the product AMP will require two phosphorylation steps to reconstitute the high-energy adenosine triphosphate, ATP. Inositol 1,4,5-triphosphate is an important molecule in the cytosol, where it releases calcium ions from storage. It forms part of a series of inositol-phosphate species that mediate calcium ion concentrations inside and outside the cell. 

Takeuchi H, Kanematsu T, Misumi Y, et al. Distinct specificity in the binding of inositol phosphates by pleckstrin homology domains of pleckstrin, RAC-protein kinase, diacylglycerol kinase and a new 130 kDa protein. Biochem Biophys Acta 1997 1359(3) 275-285. 

An additional action of Li" is interruption of the phosphatidylinositide cycle through an inhibitory action on inositol phosphate metabolism. By this mechanism, depletion of membrane inositol and the phosphoinosi-tide-derived second-messenger products diacylglycerol and inositol triphosphate ultimately reduces signaling through receptor systems dependent on the formation of these products. It is presently unclear to what extent inhibition of inositol phosphate metabolism contributes to the therapeutic properties of Li+ in bipolar patients. 

Maslanski JA, Leshko L, Busa WB lithium-sensitive production of inositol phosphates during amphibian embryonic mesoderm induction. Science 256 243-245, 1992... 

The inositol phosphates are linked into a metabolic cycle (Fig. 6.5) in which they can be degraded and regenerated. Via these pathways, the cell has the ability to replenish stores of inositol phosphate derivatives, according to demand. Ptdins may be regenerated from diacylglycerol via the intermediate levels of phosphatidic acid and CDP-glycerol. 

An effective treatment for bipolar disorder (manic -depressive illness) is the administration of lithium salts 445/1111-11133 Inhibition of the hydrolysis of inositol phosphate by Li+ (Fig. 11-9) may be related to its therapeutic effect. Reduced phosphatidylinositol turnover may dampen responses to neurotransmitters.1114 Li+ may affect gene expression in neuropeptide-secreting neurons.1115 Bipolar disorder apparently has more than one cause. There are strong indications of genetic susceptibility,1116 and genes that increase susceptibility have been located on chromosomes 4,12,13,18,21, and X.1117... 

S. V. Ley, Stereoselective synthesis of inositol phosphates, Pure Appl. Chem. 1990, 62, 2031-2034. 

Brown, EM, Fuleihan, Ge-H, Chen, CJ and Kifor, O, 1990, A comparison of the effects of divalent and trivalent cations on parathyroid hormone release, 3, 5 -cyclic-adenosine monophosphate accumulation, and the levels of inositol phosphates in bovine parathyroid cells, Endocrinology 127 1064-1071... 

To obtain model compounds for the study of inositol phosphates, Kil-gour and Ballou180 treated di-O-isopropylidenepinitol (LXIV) and di-O-isopropylidene-Zeyo-inositol (XXVIII) with diphenyl phosphorochloridate. For the latter, the result was surprising instead of the expected mono-or di-phosphate, a cyclic phosphate (LXXIX) was obtained. In it, the phosphate group is attached to two irans-hydroxyl groups, like the third iso-propylidene group in tri-O-isopropylidene-fevo-inositol (see p. 149). This... 

The functional significance of the various edited isoforms of the 5-HT2C receptor has been underscored by their differential abundances of expression in total brain and hypothalamic mRNA populations (293) and distinct functional properties. Moreover, the variant edited receptors exhibit differential abilities to bind various ligands, to mobilize intracellular Ca2+, and to stimulate hydrolysis of inositol phosphates (232,292,293). Bums and colleagues showed that mRNA editing of the 5-HT2C receptor leads to a 10- to 15-fold reduction in the efficacy of the interaction between receptors and their... 

Shubieta, H.E., McDonough, PM., Harris, A.N., Knowlton, K.U., Glembotski, C.C., Brown, J.H., and Chien, K.R. 1990. Endothelin induction of inositol phosphate hydrolysis, sarcomere assembly, and cardiac gene expression in ventricular myocytes. A paracrine mechanism for myocardial cell hypertrophy. J.Biol. Chem. 265 20555-20562. 

If one is interested in the specific facets of the above techniques and procedures, two recommended publications for reading would be that of Trayner-Kaplan et al. (1988) and that of Stephens et al. (1991). Further general information on the chemistry and biochemistry of inositol phosphates can be found in a (symposium) publication edited by Reitz (1991). 

Dean, N. M. and Moyer, J. D. (1987) Separation of multiple isomers of inositol phosphate formed in GH3 cells, Biochem. J. 242, 361-366. 

The formation of inositol phosphates, including inositol 1,4,5-trisphosphate (IP3) has been demonstrated in ovarian cells [13,14], The levels of these compounds are raised for periods of seconds to minutes after addition of LH to isolated cells. However, these LH-induced changes are an order of magnitude smaller than the changes in cyclic AMP. The transient and small effects of LH on polyphosphoinositide turnover has made it difficult to establish accurate kinetics and dose response charac-... 

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