Thermospray liquid chromatographic/mass spectrometric studies with inositol phosphates

Analysis of glycerophosphoinositol by liquid chromatography–electrospray ionisation tandem mass spectrometry using a β-cyclodextrin-bonded column

Glycerophosphoinositol (GroPIns) has been demonstrated to have important roles in many intracellular regulatory processes. GroPIns has been analysed for many years by anion-exchange HPLC after radiolabelling of cells in culture, but no method has been developed, to our knowledge, for the direct detection and quantitation of the unlabelled compound in such biological samples. Here is reported a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the direct quantitative analysis of GroPIns that can indeed be applied to cell extracts. Analyses were performed on a beta-cyclodextrin-bonded HPLC column using a binary mobile phase of acetonitrile and 20 mM ammonium formate in water, which allowed direct on-line detection by tandem mass spectrometry in negative electrospray ionisation (ESI) mode. The method was applied to the quantitative analysis of GroPIns in selected rat cell lines after a two-phase acid extraction of cultured cells using external calibration. The potential matrix signal suppression effects were investigated by the parallel quantitation of GroPIns in extracts of selected cultured cell lines with both external calibration and the standard additions method. The accuracy data obtained demonstrated the feasibility of external calibration, so allowing a simpler and less time-consuming approach than that of the standard additions method.

Structural distinction among inositol phosphate isomers using high-energy and low-energy collisional-activated dissociation tandem mass spectrometry with electrospray ionization

Electrospray (ESI) collisional-activated dissociation (CAD) tandem mass spectrometric methods for the structural characterization of inositol phosphates (InsPs) using both quadrupole and sector mass spectrometers are described. Under low-energy CAD, the [M + H](+) ions of the positional isomers of inositol phosphates, including inositol mono-, bis- and trisphosphates, yield distinguishable product-ion spectra, which are readily applicable for isomer differentiation. In contrast, the product-ion spectra arising from high-energy CAD (2 keV collision energy, floating at 50%) tandem sector mass spectrometry are less applicable for isomer identification. The differences in the product-ion spectrum profiles among the aforementioned InsP isomers become more substantial and differentiation of positional isomers can be achieved when the collison energy is reduced to 1 keV (floating at 75%). These results demonstrate that the applied collision energies play a pivotal role in the fragmentations upon CAD. The product-ion spectra are similar among the positional isomers of inositol tetrakisphosphates and of inositol pentakisphosphates. Thus, isomeric distinction for these two inositol polyphosphate classes could not be established by the tandem mass spectrometric methods that have achieved such distinctions for the less highly phosphorylated inositol phosphate classes. Under both high- and low-energy CAD, the protonated molecular species of all InsPs undergo similar fragmentation pathways, which are dominated by the consecutive losses of H(2)O, HPO(3) and H(3)PO(4).

Inositol phosphates in the environment

The inositol phosphates are a group of organic phosphorus compounds found widely in the natural environment, but that represent the greatest gap in our understanding of the global phosphorus cycle. They exist as inositols in various states of phosphorylation (bound to between one and six phosphate groups) and isomeric forms (e.g. myo, D-chiro, scyllo, neo), although myo-inositol hexakisphosphate is by far the most prevalent form in nature. In terrestrial environments, inositol phosphates are principally derived from plants and accumulate in soils to become the dominant class of organic phosphorus compounds. Inositol phosphates are also present in large amounts in aquatic environments, where they may contribute to eutrophication. Despite the prevalence of inositol phosphates in the environment, their cycling, mobility and bioavailability are poorly understood. This is largely related to analytical difficulties associated with the extraction, separation and detection of inositol phosphates in environmental samples. This review summarizes the current knowledge of inositol phosphates in the environment and the analytical techniques currently available for their detection in environmental samples. Recent advances in technology, such as the development of suitable chromatographic and capillary electrophoresis separation techniques, should help to elucidate some of the more pertinent questions regarding inositol phosphates in the natural environment.

Studies on the permethylation/dephosphorylation of inositol polyphosphates: an approach to a more sensitive assay

Methods for the permethylation of inositol phosphates (i.e. the formation of the completely substituted C-O-methyl/P-O-methyl derivatives) have been studied as a precursor to preparing C-O-methyl inositols where the remaining inositol hydroxyl groups are at the positions originally occupied by the phosphomonoesters. Classical sodium-driven methylations, diazomethane methylations and methylation with methyl trifluoromethanesulfonate were studied and only the latter was found to produce completely alkylated inositol phosphates. Treatment of the permethylated substrates with methanolic HCl removed the dimethylphosphate groups to produce C-O-methyl inositols which are candidates for negative ion chemical ionization gas chromatographic/mass spectrometric analysis as heptafluorobutyryl C-O-methyl inositols. As an example, gas chromatographic/mass spectrometric analysis of myo-inositol 1,2,6-trisphosphate was carried out by methylation, dephosphorylation and conversion to the tris(heptafluorobutyryl) derivative. Detection at the low-femtomole level was achieved by this means. A limitation of the method may be that the methylation procedure appears to produce a variable degree of phosphate positional isomerization, with resulting loss of specificity. If stable isotope internal standards were available for the inositol polyphosphates of interest, this limitation could be compensated for.