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PubMed2001

Atmospheric pressure chemical ionization mass spectrometry for analysis of lipids.

Atmospheric pressure chemical ionization (APCI) mass spectrometry (MS) has proven to be a very valuable technique for analysis of lipids from a variety of classes. This instrumental method readily produces useful ions with gentle fragmentation from large neutral molecules such as triacylglycerols and carotenoids, which are often difficult to analyze using other techniques. Molecules that are easily ionized, such as phospholipids, produce molecular ions and diagnostically useful fragment ions that are complementary to those produced by methods such as electrospray ionization MS with collision-induced dissociation. The simplicity and versatility of APCI-MS make it an ideal tool for use in solving hitherto very difficult analytical problems.

Animals↗

PubMed1997

Identification of milk fat triacylglycerols by capillary supercritical fluid chromatography-atmospheric pressure chemical ionization mass spectrometry.

Identification of milk fat triacylglycerols was accomplished by capillary supercritical fluid chromatography (SFC) combined with atmospheric pressure chemical ionization mass spectrometry [(APCI)MS]. Supercritical carbon dioxide was the carrier fluid in SFC. Ionization was achieved by introducing vapor of ammonia in methanol into the ionization chamber which resulted in the formation of abundant [M + 18]+ and [M - RCCO]+ ions of triaclyglycerols. These ions defined both the molecular weight and the fatty acid constituents of a triacylglycerol, respectively. SFC on a nonpolar stationary phase provided an efficient separation of triacylglycerols according to the combined number of carbon atoms in the acyl chains a molecule. In addition to the identification of the major chromatographic peaks representing molecules with 26-54 acyl carbons, minor peaks representing triacylglycerols with an odd number of acyl carbons were separated and identified. Furthermore, compositional information on partially separated isobaric triacylglycerols, which differed substantially in the chain length of the fatty acyl residues, was achieved within some of the peaks. A new finding of the present study was the formation of abundant [M + 18]+ ions of saturated triacylglycerols in addition to diagnostic fragment ions,being of primary importance in structure elucidation. This extends the applicability of capillary SFC-(APCI)MS in the analysis of both saturated and unsaturated triacylglycerols.

Animals↗

PubMed1987

Lack of evidence of low ionized calcium levels in systemic hypertension.

An epidemiologic screening survey was conducted in 325 male industrial workers to investigate the relation between serum total and ionized calcium concentrations and blood pressure. No relation was found. Previous reports of lower serum ionized calcium levels in hypertensive patients may be related to methodologic deficiencies both in the selection of subjects and in ionized calcium measurement. These data do not support the concept that increased blood pressure levels are related to calcium deficiency or to abnormal plasma calcium homeostasis.

Adult↗

PubMed1986

Comparison of electron and chemical ionization mass spectrometry of sialic acids.

Sialic acids were analyzed as per-O-trimethylsilylated compounds by gas-liquid chromatography/mass spectrometry either on electron or chemical ionization by isobutane. Electron ionization mass spectra of these derivatives are very similar to those of the corresponding methyl esters described earlier whereas chemical ionization mass spectra are characterized in the high mass range by loss of the C-2 and the C-4 substituents from the M + 1 ion. Together with other fragment ions the seven different sialic acids analyzed could be clearly identified.

Esters↗

PubMed1988

Measurement of ionized calcium in normal human blood platelets.

To measure the concentrations of cytosolic ionized calcium in platelets we used a calcium-sensitive fluorophor (Fura-2) with two different spectrofluorometers (Perkin-Elmer LS-5 and Fluorolog 222). Values obtained by these two instruments for the basal cytosolic ionized calcium concentration of resting platelets and those of agonist-activated platelets did not differ significantly. Both instruments were capable of monitoring the shifts in wavelengths induced by the dye-calcium complex, the ratio between absorbances at the two wavelengths (340/380 nm), and calcium concentrations continuously during agonist-induced platelet activation. We conclude that a relatively inexpensive instrument may be adequate for measuring ionized calcium in cells by this method, although sophisticated kinetic studies may require analytical or research-grade instruments.

Adult↗

PubMed1989

Boronic acids for affinity chromatography: spectral methods for determinations of ionization and diol-binding constants.

Arylboronic acids attached to solid matrices have proved useful for the diol-specific chromatography of biomolecules and affinity purification of enzymes by exchangeable-ligand chromatography. The latter use has been limited by the intrinsic ionization constant (pKa approximately 9) of the most common commercial products. The synthesis of several arylboronic acids with ionization constants near neutrality are described, and the application of a new general, spectral-difference method for determining acid ionization constants and formation constants with fructose is developed. In particular 4-(N-methyl) carboxamido-benzeneboronic acid was found to have a pKa of 7.86 and a formation constant with D-fructose of 8600. It was stable toward acid or base hydrolysis. We suggest that 4-carboxybenzeneboronic acid might be useful for preparing matrices for enzyme affinity chromatography.

Boronic Acids↗

PubMed1989

Analysis of histamine and N tau-methylhistamine in plasma by gas chromatography-negative ion-chemical ionization mass spectrometry.

Current methods of quantitation of histamine and its major metabolite N tau-methylhistamine are inaccurate and insensitive to the very low concentrations that exist in plasma samples. Therefore, an accurate and sensitive method for quantification in plasma has been developed using the stable isotope dilution assay with negative ion-chemical ionization mass spectrometry. For histamine, after the addition of [2H4]histamine to 2 ml of plasma, the plasma sample is deproteinized, extracted into butanol, back extracted into HCl, derivatized to the pentafluorobenzyl derivative (CH2C6F5)3-histamine, purified on silica gel columns, and then quantified with negative ion-chemical ionization mass spectrometry by selected ion monitoring of the ratio of ions m/z 430/434. For N tau-methylhistamine, after the addition of N tau-[2H3]methylhistamine to 2 ml of plasma, the plasma sample is deproteinized, extracted into butanol, back extracted into HCl, derivatized to the heptafluorobutyryl derivative (C3F7CO2)2-N tau-methylhistamine, purified on silica gel columns, and then quantified with negative ion-chemical ionization mass spectrometry by selected ion monitoring of the ratio of ions m/z 497/500. The precision of the histamine assay is 3.1% and the accuracy is 95.5 +/- 2.5% while the precision of the N tau-methylhistamine assay is 1.9% and the accuracy is 106.8 +/- 1.9%. The lower limits of sensitivity are 1 pg for histamine and 6 pg for N tau-methylhistamine injected on column. Using the assay in three normal human volunteers, plasma concentrations of histamine were 130, 92, and 85 pg/ml, and of N tau-methylhistamine were 229, 228, and 216 pg/ml. This assay provides a very sensitive and accurate method of quantitation of histamine and N tau-methylhistamine in plasma samples.

Animals↗

PubMed1983

Ionization-linked cooperative interactions in the active site of horse liver alcohol dehydrogenase.

Affinity labeling of horse liver alcohol dehydrogenase with iodoacetate in the presence of the activator imidazole has been studied from pH 6.1 to 10.5. The pH profiles for the dissociation constants of iodoacetate from the free enzyme and the enzyme-imidazole complex and of imidazole from the free enzyme and the binary enzyme-iodoacetate complex were determined. The variation with pH of the dissociation constants of iodoacetate (KI) and imidazole (KL) have in common a pKa of 8.6 assigned to the zinc-water ionization, and a pKa near 10. Lysine modification by ethyl acetimidate results in a higher affinity of iodoacetate to the enzyme at high pH as the pKa values of the lysine residues are increased. The binding of iodoacetate and imidazole at each enzyme subunit shows negative cooperativity at pH less than 9, with an interaction constant of 4.8 at pH 6.1. Positive cooperativity is observed at pH greater than 9, with an interaction constant of 0.5 at pH 10.5. The pH-dependent change in cooperativity results from the removal of the zinc-water ionization when imidazole becomes coordinated to the catalytic zinc ion. When iodoacetate binds at the anion binding site, a large perturbation of the zinc-water ionization is observed. Unlike imidazole, the binding of 1,10-orthophenanthroline and iodoacetate shows positive cooperativity at both pH 8.2 and 10.0 with an interaction constant as low as 0.06 at pH 10.0.

Alcohol Dehydrogenase↗

PubMed1991

The ionization states of the 5'-phosphate group in the various coenzyme forms bound to mitochondrial aspartate aminotransferase.

We have carried out a Fourier transform infrared spectroscopic study of mitochondrial aspartate aminotransferase in the spectral region where phosphate monoesters give rise to absorption. Infrared spectra in the above-mentioned region are dominated by protein absorption. Yet, below 1020 cm-1 protein interferences are minor, permitting the detection of the band arising from the symmetric stretching of dianionic phosphate monoesters [T. Shimanouchi, M. Tsuboi, and Y. Kyogoku (1964) Adv. Chem. Phys. 8, 435-498]. The integrated intensity of this band in several enzyme forms (pyridoxal phosphate, pyridoxamine phosphate, and sodium borohydride-reduced, pyridoxyl phosphate form) does not change with pH in the range 5-9. This behavior contrasts that of free pyridoxal phosphate (PLP) and pyridoxamine phosphate (PMP) in solution, where the dependence of the same infrared band intensity with pH can be correlated to the known pK values for the 5'-phosphate ester in solution. The integrated intensity value of this infrared band for the PLP enzyme form before and after reduction with sodium borohydride is close to that given by free PLP at pH 8-9. These results are taken as evidence that in the active site of mitochondrial aspartate aminotransferase the 5'-phosphate group of PLP remains mostly dianionic even at a pH near 5. Thus, it is suggested that the chemical shift changes associated with pH titrations of various PLP forms reported in a previous 31P NMR study of this enzyme [M. E. Mattingly, J. R. Mattingly, and M. Martinez-Carrion (1982) J. Biol. Chem. 257, 8872] are due to the fact that the phosphorus chemical shift senses the O-P-O bond distortions induced by the ionization of a nearby residue. Since no chemical shift changes were observed in pH titrations of the PMP forms (lacking an ionizable internal aldimine) of this isozyme, the Schiff base between PLP and Lys-258 at the active site is the most likely candidate for the ionizing group influencing the phosphorus chemical shift in this enzyme.

Animals↗

PubMed1989

Experimental values of the ionization constants for L-3,5-di-iodotyrosine and a model for ionic interactions of thyroid hormone (T3) and its nuclear receptor.

Ionization characteristics of L-3,5-di-iodotyrosine have been measured under various conditions. These data, which correct an erroneous report of pK3 in the literature, have been used to estimate the ionization constants of the thyroid hormone L-3',3,5-tri-iodothyronine (T3). On this basis a reinterpretation has been made of the pH-dependent binding of the hormone to its solubilized rat liver nuclear receptor (Wilson BD and Gent WL, Biochem J 232: 663-667, 1985). The interaction may depend on the ionization of the phenolic group of T3 and an acidic group (pK'7.6) in the receptor site, leading to the formation of a hydrogen bond between the two groups. The changes of the number of binding sites with pH must then result independently from alterations in the conformation of the receptor protein.

Diiodothyronines↗

PubMed1984

Chemical-ionization mass spectra of the permethylated sialo-oligosaccharides liberated from gangliosides.

Permethylated mono- and di-sialo-oligosaccharides liberated from several parent gangliosides have been examined by chemical-ionization mass spectrometry with ammonia as the reagent gas in order to elucidate their structures. Several major fragment-ions, in addition to both the protonated and ammonium adduct molecular-ions, may be readily assigned without interference from the ceramide moiety. Sialic acid-containing di-, tri-, and tetra-saccharide ions can be clearly observed and used to determine the sugar residue to which the sialic acid residue is attached. The neutral-sugar skeletons produced by the loss of sialic acid give rise to both the protonated and the ammonium adduct ions; in the case of tetrasaccharides, these are further degraded to produce di- and tri-saccharide ions. These characteristic ions are useful for the determination of the number of sugar residues and their sequence in an oligosaccharide structure. The chemical-ionization mass spectra of GM3- and GM1-oligosaccharides with isobutane show the ions corresponding to each monosaccharide residue. These results indicate that chemical-ionization mass spectrometry is highly useful in determining the complete sugar-sequence of gangliosides.

Brain Chemistry↗

PubMed1993

Mutagenicity of formaldehyde in Chinese hamster lung fibroblasts: synergy with ionizing radiation and N-nitroso-N-methylurea.

Cultured Chinese hamster V79 cells, a widely utilized model system in risk assessment of environmental agents, have been utilized to measure toxicity and mutagenicity of formaldehyde with or without previous exposure to either the alkylating agent N-nitroso-N-methylurea or to ionizing radiation. Each of these agents caused a dose-dependent decrease in colony forming efficiency and a parallel increase in 6-thioguanine resistant colonies. Significant mutant frequencies were induced by 0.3 up to 1 mM formaldehyde, 2 and 4 Gy of radiation and 0.2 and 0.5 mM N-nitroso-N-methylurea. Exposure of cells to ionizing radiation or N-nitroso-N-methylurea followed by submutagenic concentrations of formaldehyde potentiated both the cytotoxicity and the mutagenicity as compared with the corresponding separate effects caused by each of these agents. Taken together, these studies clearly demonstrate genotoxic effects in vitro of three recognized carcinogens, i.e. formaldehyde, N-nitroso-N-methylurea and ionizing radiation. Moreover, the synergies now demonstrated in regards to cytopathic consequences indicate interactive effects between formaldehyde and these agents, representing both a chemical and a physical carcinogen.

Animals↗

PubMed1977

Combined gas chromatography-chemical ionization mass spectrometry of sphingolipids. I. Glucosyl sphingosine, ceramides and cerebrosides of the spleen in Gaucher's disease.

Trimethylsilylated glucosyl sphingosine, ceramides and glucocerebrosides were analysed by combined gas chromatography (GC)-chemical ionization (CI) mass spectrometry. Isobutane, methane and ammonia were used as reactant gasses for chemical ionization. Essentially the same fragment ions were detected in the spectra with the different reactant gases. Purified glucocerebrosides isolated from the spleen of a patient with Gaucher's disease were clearly separated into their 8 molecular species by gas chromatography. Three other minor components were detected by spectrometry, and these 11 molecular species of glucocerbrosides from the spleen of the patient with Gaucher's disease have been analysed. The ceramides obtained by periodate oxidation and then alkaline reduction of the glucocerebrosides were also separated into 11 molecular species by GC-CI mass spectrometry. Molecular weight could be determined using the major fragment ion of m/e (M+-90) in the spectra of ceramides and cerebrosides. The chemical ionization method is useful for structural analysis and determination of the molecular species of sphingoglycolipids.

Adult↗

PubMed1978

A simple screening method for plasma lipids by thin-layer chromatography with flame ionization detection.

Thin-layer chromatography was carried out on glass rods coated with Silica gel (chromarod) and the fractions were detected with a flame ionization detector (FID). This system was applied to the fractionation and screening of plasma lipids. As the relative response of the flame ionization detector depends on the nature of the lipids, correction factors had to be introduced. These correction factors were derived from comparison of the results of the thin-layer chromatography with flame ionization detection with those obtained by conventional chemical analysis of the same sample. The coefficients of variation of the lipid profiles were comparable for both techniques. The absolute lipid concentration for the various fractions obtained by combination of the concentration of the total lipids with the relative lipid profile, correlate very well with the values obtained by conventional chemical analysis of the various lipids.

Cholesterol↗

PubMed1980

Determination of ionized calcium in parotid saliva.

A new method for the determination of ionized calcium in parotid saliva utilizing an automatic ionized calcium analyzer (Orion Biomedical Model Space-Stat 20) was evaluated. The method showed good precision with a relative standard deviation of 2.9--3.8 per cent in the range 0.20--0.79 mmol . l-1 after 226 duplicate measurements. Mean recovery was 94.9% in the range of 0.6--1.8 mmol. l-1. Bovine mucin was found not to interfere with the calcium electrode. Differences in sodium concentration, ionic strength and pH between the standard solutions and samples caused systematic errors. These were corrected using experimentally derived formulas. The influence of pH on the concentration of ionized calcium was studied in 13 parotid saliva samples. A non-linear relationship ranging between 32 and 84% of the total calcium concentration was found.

Animals↗

PubMed1995

Application of a new atmospheric pressure ionization source for double focusing sector instruments.

In three application examples we demonstrate how the capabilities of a modern atmospheric pressure ionization (API) source, as a universal LC-MS coupling tool, are supplemented by the high mass resolution available on double focusing sector instruments. The examples include charge state determination in complex electrospray ionization-collisional induced dissociation (ESI-CID) spectra of multiply charged ions, mixture analysis of multicomponent samples without prior chromatographic separation, and very sensitive and highly specific detection of pesticides using atmospheric pressure chemical ionization (APCI).

Amino Acid Sequence↗

PubMed1989

Ionized and wobble base-pairing for bromouracil-guanine in equilibrium under physiological conditions. A nuclear magnetic resonance study on an oligonucleotide containing a bromouracil-guanine base-pair as a function of pH.

A one and two-dimensional nuclear magnetic resonance study of a non-selfcomplementary oligonucleotide containing a central 5-bromouracil-guanine pair is reported. For these two bases three types of hydrogen bonding schemes could exist; wobble, rare tautomer and ionized. The two-dimensional spectra of non-exchangeable protons together with one-dimensional spectra recorded in water show that at pH 7.0 the predominant species is a right-handed B-form DNA in which the brU.G pair has wobble geometry. On raising the pH we observe a transition monitored by proton chemical shift changes for the brU.G and adjacent base-pairs. The mid-point of the transition was observed at pH 8.6. Spectra recorded at pH 9.8 show that the helix remains intact with B form conformation. It is shown that this high pH form has an ionized brU.G base-pair now in Watson-Crick geometry. Thus under physiological conditions an equilibrium exists between wobble and ionized structures.

Base Composition↗

PubMed1984

Possible mechanism of uptake for several compounds in ionized form through human erythrocyte membrane.

The mechanism underlying uptake of certain compounds in ionized form across human red blood cell membrane was examined. The ionized forms of salicylate, 5-methoxy-salicylate and phenylalanylphenylalanine were significantly taken up into the interior space of human red blood cells instead of remaining in the membrane. Inhibition of the uptake of these compounds by 4,4'-diisothiocyano-2,2'-disulfonate stilbene and phlorizin indicates that their permeation of the erythrocyte membrane may involve the membrane protein fraction. Chelation at the protein site does not appear to occur. Instead, an amino group in the protein structure may mediate the transport of these ionized compounds.

Absorption↗