Assessment of metals in reconstituted metallothioneins by electrospray mass spectrometry

A method has been developed that combines electrospray ionization mass spectrometry with pH control to provide analysis of metals in native or reconstituted metallothioneins. These metalloproteins cooperatively bind seven divalent metal ions, most commonly Zn2+ and Cd2+. Since the protein is denatured and metal ions are lost below pH3, the pH of the electrospray solution is critical to successful results. The metal-free apoprotein was detected with its most abundant ions in a charge state of 6+, while the folded metallothionein-metal complexes were observed with lower charge states. The retention of seven metals in the molecular ions detected is consistent with the hypothesis that metallothionein retains its conformation in the gas phase. This mass spectrometric technique can be used to determine rapidly and accurately how many and what cations are incorporated per molecule of protein. Information about molar distributions and estimates of relative abundances of various complexes in the sample can be acquired in a single measurement.

Mass spectrometric analysis of proteins

The past year has seen greatly increased acceptance and application of the analytical capabilities of mass spectrometry by the biochemical community. The technique has been used to provide accurate mass determinations of non-covalently bound protein complexes, rapid mapping of molecular weights of altered peptides in protease digests, sequencing by collisional activation in tandem mass spectrometry, characterization of glycosylation and other modifications, and quantitation of peptides used in clinical diagnostics.

Role of glutathione in cellular resistance to alkylating agents

Both elevated glutathione levels and increased activity of the enzyme glutathione S-transferase have been associated with the resistance of cells to alkylating agents. We have demonstrated that one mechanism of this resistance is the inactivation of the alkylating agents by conjugation with glutathione. This conjugation can be catalyzed by glutathione S-transferase. For the nitrogen mustard agents we have studied, both the spontaneous and enzyme catalyzed reactions proceed through the aziridinium intermediates of the alkylating agents, and the alpha isoenzymes of GST are involved. In a study of cyclophosphamide resistant medulloblastoma cell lines elevated cellular concentrations of glutathione correlated well with the resistance of the cell lines.

Proton affinities of polyglycines assessed by using the kinetic method

The proton affinities of a series of peptides, chosen to show the effects of chain length, were measured by the kinetic method using amines as standard reference bases. Proton affinities of polyglycines with residues ranging from 2 to 10 were measured and the values were found to increase as the number of residues increases.

Fragmentation characteristic of glutathione conjugates activated by high-energy collisions

Product ion spectra of fifteen monoglutathione and diglutathione conjugates have been measured using activation by 6000-eV collisions with helium in the third field-free region of a four-sector tandem mass spectrometer of EBEB configuration. Fragmentation patterns in the cation spectra have been analyzed for decompositions of the glutathione moiety that would permit recognition of an unknown as a glutathione conjugate. Five spectra from an earlier study of high-energy collisional activation on a BEEB four-sector instrument have also been included in this analysis. A suite of appropriate ions was found to occur consistently,, including ions of m/z 307 comprising the glutathione tripeptide and the complementary ion [MH-307](+) or the ion radical [MH-306](+).

High-performance tandem mass spectrometry in metabolism studies

1. High-performance tandem mass spectrometry provides unit resolution in both selection of precursor ions and analysis of fragment ions, and extensive and reproducible fragmentation through collisional activation at high energy. 2. Metabolites can be analysed that occur as minor components in h.p.l.c. peaks or other mixtures. Homogeneous isotopic species can be selected for unambiguous analysis of distributions of isotope labels. Fragmentation may be significantly enhanced to provide structural information. Overall, the signal to noise ratio is greatly improved and the spectrum is simplified. 3. These points are illustrated by isotope-labelling studies of the mechanisms of glutathione conjugation of the anti-tumour agent cyclophosphamide, the cytotoxic agent phosphoramide mustard and dimethylbilirubin, an analogue of bilirubin designed to be distinguishable from endogenous bilirubin. Analysis of isomeric mixed disulphides formed between glutathione and a peptide with an internal disulphide bond is discussed. 4. Reaction-induced decomposition is presented as an alternative to collisionally induced decomposition with more efficient energy transfer.

Gag proteins of the highly replicative MN strain of human immunodeficiency virus type 1: posttranslational modifications, proteolytic processings, and complete amino acid sequences

The MN strain of human immunodeficiency virus type 1 was grown in H9 cells, concentrated by centrifugation, and disrupted, and proteins were purified by reversed-phase high-pressure liquid chromatography. Complete amino acid sequences were determined for the mature Gag proteins, showing natural proteolytic cleavage sites and the order of proteins (p17-p24-p2-p7-p1-p6) in the Gag precursors. At least two sequence variants of p24 and eight sequence variants of p17 were detected. The two most abundant variants of p24 and p17 represented at least 50% +/- 5% and 20% +/- 5% of their totals, respectively. These data suggest heterogeneity in the virus population, with 50% of the total virus containing the most abundant forms of p17 and p24 and 20% of the virus containing the second most abundant forms. The Gag precursors of these suggested viruses differ from each other by only 3 amino acid residues but differ from the precursors predicted by the published MN proviral DNA sequence by 10 residues. Electrospray ionization mass spectrometry analysis of the purified p24 forms showed that the measured molecular weight of the protein was 200 +/- 50 atomic mass units greater than the calculated molecular weight. The source of additional mass for the p24 forms was not determined, but the observation is consistent with previous suggestions that the protein is phosphorylated. Greater than 98% of the total recovered p17 was myristylated at the N-terminal glycine residue, and the measured molecular weights (as determined by electrospray ionization mass spectrometry) of the most abundant forms were within 3 atomic mass units of the calculated molecular weights (15,266).

Enhancement of biological activity by conjugation reactions

Conjugation reactions, such as the formation of sulfates, glucuronides and amino acid derivatives of biologically active compounds, are classically considered as detoxication processes. In essence, common nonpolar compounds are rendered less active by the formation of water-soluble metabolites, which then can readily be excreted in the urine. In the past decade, however, it has become clear that some natural metabolites become more active rather than less so by the formation of such conjugates. Several compounds in this special group will be considered in this article; namely, glucuronide and hydroxyphenylamide conjugates of retinoic acid, glutathione and cysteine conjugates of halogenated hydrocarbons, glucuronide conjugates of various xenobiotics and polyglutamates of folic acid and its inhibitors. The metabolism, biological activity and toxicity of the parent molecule is profoundly affected in complex ways by such reactions. Thus, the historical concept of conjugation reactions as general detoxication processes is no longer tenable.

Conjugation of polychlorinated agrochemical sulphoxides and sulphones by glutathione

1. Pentachlorophenyl methyl sulphoxide and pentachlorophenyl methyl sulphone were found to be substrates for microsomal and cytosolic glutathione-S-transferase of rabbit, monkey, chicken and human liver, covalently immobilized on beaded sepharose. 2. Protein was immobilized with greater than 95% transferase activity, measured by dinitrochlorobenzene. Immobilized rabbit liver microsomal transferase activity was more stable than immobilized cytosolic activity. 3. The sulphoxide moiety was displayed by glutathione in the presence of chicken liver microsomal protein. The sulphone moiety was displayed by glutathione in the formation of a diglutathione under catalysis by rhesus monkey liver cytosolic and microsomal protein. 4. Chlorine was displaced by transferases from all species to form regioisomeric monoglutathiones. 5. Qualitative and quantitative differences were observed in product distributions between species and between microsomal and cytosolic protein.

In vitro formation of glutathione conjugates of the dimethylester of bilirubin

Rat hepatic microsomes catalyzed the formation of two distinct glutathione conjugates of bilirubin dimethylester (DMB). The two conjugates were identical to those isolated from the bile of Gunn rats infused with DMB. The microsomal reaction was dependent on NADPH, oxygen and glutathione and was inhibited by nitrogen and the cytochrome P450 inhibitors metyrapone, 1-benzyl-imidazole, and alpha-naphthoflavone. Conjugate formation was inducible with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) but not phenobarbital pretreatment. The rate of formation of conjugates was not affected by washings of the microsomal pellet or by the presence of superoxide dismutase and/or catalase. Cation fast atom bombardment mass spectrometry (FAB/MS) of the conjugates indicated a molecular ion of 937 atomic mass units (amu). Fragmentation revealed a loss of 307 amu, consistent with glutathione, and a residual mass of 629 amu suggesting a hydroxylated derivative of DMB (612 amu). Cation FAB/MS/MS of conjugates formed in vitro under an atmosphere of oxygen-16 and oxygen-18 demonstrated the incorporation of molecular oxygen by a difference of 2 amu in the respective molecular ions. Our results suggest that DMB is oxidized by the cytochrome P450 IA gene family to an epoxide intermediate which is then subsequently conjugated with glutathione.

Identification of glutathione conjugates of the dimethyl ester of bilirubin in the bile of Gunn rats

The Gunn rat, which is deficient in the UDP-glucuronosyltransferase for bilirubin, promptly excreted polar conjugates of the dimethyl ester of bilirubin in bile after intravenous infusion of this ester. The conjugates proved to be monoglutathione thioether adducts of the vinyl groups of the parent tetrapyrrole. High performance liquid chromatographic analysis of the conjugates as their dipyrrolic azosulfanilates demonstrated that only one of the dipyrroles of each tetrapyrrole was conjugated. The nonconjugated dipyrrole eluted as either the methyl endo- or exovinyl azodipyrrole. The amino acid composition of the pigments was consistent with that of a monoglutathione conjugate. NMR spectroscopy of the two major pigments demonstrated the loss of the proton signals of the C-18 vinyl group, indicating it to be the site of conjugation. Cation fast atomic bombardment tandem mass spectrometry demonstrated a molecular ion, [M + H]+, of m/z 937, which fragmented with a loss of 307 atomic mass units, consistent with glutathione. A molecular ion of m/z 807 was observed for the conjugate treated with gamma-glutamyltranspeptidase, consistent with the loss of glutamate. The mass spectrometry data indicated that the conjugates also contained a functional group whose mass was equivalent to hydroxyl, suggesting initial formation of an epoxide, which then reacts with glutathione. Pretreatment of the rat with 2,3,7,8-tetrachlorodibenzo-p-dioxin to induce cytochrome P-450 resulted in a 6-fold increase of the biliary excretion of the glutathione conjugates. Such induction also resulted in the excretion of a glutathione conjugate of bilirubin itself.

Disulfide bonds in native and recombinant fish growth hormones

Disulfide linkages were characterized for the first time in a fish growth hormone. Trypsin digestion of chum salmon growth hormone, followed by mass spectrometry established that Cys-49 is linked to Cys-161, while Cys-178 is linked to Cys-186. This is analogous to the big loop, little loop pattern found in human growth hormone. Ninety-three percent of the primary structure of a recombinant rainbow trout growth hormone whose cDNA codes for the same amino acid sequence as chum salmon growth hormone was confirmed by mass spectrometric peptide mapping.

Primary structure of molluscan metallothioneins deduced from PCR-amplified cDNA and mass spectrometry of purified proteins

The primary structure of metallothioneins (MT) of a mollusc, the oyster Crassostrea virginica, was determined by molecular cloning and mass spectrometry of purified proteins. The cloning strategy included PCR amplification of the responsible cDNAs from total cDNA using completely degenerate oligonucleotides (derived from the N-terminal amino acid sequence) and oligo(dT)20 as primers. Primer extension off mRNA was used as an independent determination of the nucleotide sequence represented by the degenerate PCR primers. The deduced amino acid sequence was consistent with characteristics of class I MT. Twenty-one cysteine residues, were arranged in nine Cys-X-Cys motifs, five as Cys-Lys-Cys. A single Cys-X-X-Cys motif was also observed. Two MTs that differ only in the presence or absence of an N-acetyl group exist in this organism. Masses of tryptic peptides of purified MTs corresponded with those of peptides predicted from tryptic cleavages of the deduced amino acid sequence. Allowing for known N-terminal modifications, 96% of the deduced sequence was confirmed by mass spectrometry. Comparison (FASTA algorithm) of the primary structure of the oyster MTs with those of other species indicated a higher similarity with vertebrate MTs than with those of other invertebrates.

A mass spectrometry method for mapping the interface topography of interacting proteins, illustrated by the melittin-calmodulin system

The shielding of lysine groups from acetylation by acetic anhydride has been used to identify the regions of calmodulin in contact with melittin in the 1:1 complex. The estimation of the degree of acetylation was done by examining cyanogen bromide and cyanogen bromide/trypsin digests by mass spectrometry. Evidence was obtained that lysines-21, -75, and -148 are protected to some extent, with the implication that both the N- and C-terminal lobes and the connecting strand are involved in the interaction.

Four-sector tandem mass spectrometric analysis of complex mixtures of phosphatidylcholines present in a human immunodeficiency virus preparation

A number of phosphatidylcholines have been isolated from an HIV-1/MN preparation by reversed-phase high-performance liquid chromatography (HPLC) and analyzed by fast atom bombardment mass spectrometry (FABMS), FABMS/MS, and FABMS/MS/MS in both positive- and negative-ion modes. Negative-ion FABMS/MS with high-energy collisions was used to identify the length of the acyl groups and the degree of saturation, as well as their position on the glyceride group. FABMS/MS in the positive-ion mode was used to identify the polar head group. Negative-ion FABMS/MS/MS was used to locate positions of double bonds in acyl groups. We find that four-sector tandem mass spectrometry with high-energy collisional activation provides qualitative analysis of viral phosphatidyl lipids in considerable detail, as well as semiquantitative information. Approximate quantitation of the phosphatidylcholine content of the HIV-1/MN preparation by measuring relative peak heights of molecular ions in FABMS reveals an array of phosphatidylcholines consistent with that found in human erythrocytes, indicating the likely source of lipids in the viral membrane to be the host cell membrane.

Glutathione conjugation with phosphoramide mustard and cyclophosphamide. A mechanistic study using tandem mass spectrometry

The conjugations of cyclophosphamide and of phosphoramide mustard with glutathione are shown to be catalyzed by hepatic cytosolic glutathione-S-transferases. Cyclophosphamide conjugation is also catalyzed by microsomal glutathione-S-transferases, both in intact microsomes and after solubilization and immobilization. Deuterium isotope labels are used to test whether chloride is directly displaced by glutathione in the enzyme-catalyzed conjugations, or whether conjugation takes place via symmetrical cyclic aziridinium ions. Tandem mass spectrometry with high energy collisional activation is shown to provide reliable analysis of the isotope-labeling patterns in the conjugated products. This experiment leads to the conclusion that the aziridinium ion is opened in the conjugation of phosphoramide mustard in both the enzyme-catalyzed and the chemical reactions. Cyclophosphamide, on the other hand, is shown to be conjugated through direct displacement of chloride.

Endothermic ion molecule reactions

Endothermic ion-molecule reactions in a tandem mass spectrometer have been used for a number of years for determining thermodynamic quantities, such as heats of formation and proton affinities, for gaseous ions. Recently, the reactive, endothermic collision has been exploited as an analytical technique for the structural analysis of peptides and other biomolecules. The technique is based upon the endothermic transfer of protons associated with amide bonds to ammonia. This reaction proceeds via a long-lived collision complex. When additional beam energy is supplied, other dissociation channels are opened up, leading to the production of sequence ions for the mass-selected, protonated analyte that are normally observed in high energy collision-induced dissociation spectra. The advantage, however, is that such spectra can be produced at very low beam energies. In this article, the rationale for developing this scheme, and its roots in previous ion-molecule studies, are explored.

Recognition of trypsin autolysis products by high-performance liquid chromatography and mass spectrometry

Potential artifactual contributions are assessed in high-pressure liquid chromatograms and fast atom bombardment mass spectra from autolysis of different preparations of the widely used protease trypsin. Both commercially supplied and laboratory-purified samples were examined. Bovine pancreatic trypsin (1 mg/mL) was found to be completely destroyed in 2 h at pH 8.5, degraded to a complex mixture of small peptides which were characterized by their molecular weights. Some identifications were supported by sequencing by tandem mass spectrometry or by mass spectrometric analysis of the mixture resulting from a single Edman degradation. Autolysis of porcine pancreatic trypsin produced a completely different set of peptides. Five sites of hydrolysis at asparagine residues in bovine trypsin were also identified.

Primary sequence of duck metallothionein

Only one metallothionein appears in domesticated duck upon zinc induction. The complete amino acid sequence has been elucidated. This metallothionein has the same sequence as the chicken metallothionein, as determined by chemical sequencing of overlapping peptides produced by selective proteinase digestion and confirmed by mass spectrometry. The observation that animals of divergent origins share a common gene product presents an example of extreme conservation of a stress-inducible protein.

Oligopeptide fragmentation viewed as constant neutral loss

Spectra were recorded of all fragment ions formed by elimination of 28-u neutral fragments in fast atom bombardment spectra of peptides in the mass range 1000-1800 u, This approach can provide less complex spectra than either conventionally scanned spectra or product ion scans from collisionaIly activated four-sector experiments, and spectra that contain information that is both overlapping and complementary to those from the other techniques. Constant neutral loss spectra may provide a reading frame for distinguishing sequence ion series in tandem or single analyzer spectra.

Laser desorption electron impact: application to a study of the mechanism of conjugation of glutathione and cyclophosphamide

Toward the objective of producing ion radical species from involatile and thermally labile samples, we have combined laser desorption of neutral molecules with electron impact ionization on a time-of-flight mass analyzer with a delayed draw-out pulse. The analytical capabilities of this method are tested in the analysis of isotope labels in the involatile product in a mechanistic study of both the chemical and the enzyme catalyzed reactions of cyclophosphamide with glutathione.

Characterization of glutathione conjugates of chlorambucil by fast atom bombardment and thermospray liquid chromatography/mass spectrometry

Chlorambucil (p-(di-2-chloroethyl)amino-gamma-phenylbutyric acid) is a bifunctional alkylating agent which exhibits acquired drug resistance upon repeated dosing in humans. This compound reacts with glutathione both non-enzymatically and enzymatically in the presence of immobilized microsomal glutathione-S-transferases to produce several glutathione conjugates. These conjugates result from displacement of one or both chlorines by the nucleophilic cysteine sulfhydryl moiety of glutathione. The mono- and diglutathionyl conjugates of chlorambucil were purified by reversed-phase high-performance liquid chromatography and characterized by positive ion fast atom bombardment mass spectrometry. In addition, the mono- and dihydroxy hydrolysis products of chlorambucil were characterized by positive ion thermospray liquid chromatography/mass spectrometry (LC/MS). The glutathione conjugates of chlorambucil did not produce molecular ion species in thermospray LC/MS mode, but gave characteristic ions at m/z 147 corresponding to fragmentation of the glutathione moiety. The formation of glutathione conjugates of this class of alkylating agents may play a role in the development of acquired drug resistance.

Adaptation of a thermospray liquid chromatography/mass spectrometry interface for use with alkaline anion exchange liquid chromatography of carbohydrates

An interface is described that allows the direct coupling of high-performance alkaline anion exchange liquid chromatography with thermospray mass spectrometry. A membrane suppressor is used to remove nonvolatile alkaline salts from the mobile phase after the chromatographic process is completed and prior to introduction into the mass spectrometer. Examples are given of both isocratic and gradient separations of a three-component test mixture of N-acetylated mono- and disaccharides, followed by on-line mass spectral data acquisition. Sensitivity studies show minimum detection limits for the test compounds to be in the microgram range.

Protocol for liquid chromatography/mass spectrometry of glutathione conjugates using postcolumn solvent modification

A novel protocol for thermospray liquid chromatography/mass spectrometry (LC/MS) analysis of mixtures of glutathione conjugates is reported. Solvent conditions for optimal high-performance liquid chromatography are not always the same as for optimal thermospray ionization mass spectrometry. Labile glutathione conjugates that give poor spectra in aqueous ammonium acetate yield more intense molecular ion signals with increased percentages of acetonitrile. Direct injection thermospray ionization using 30-60% acetonitrile in aqueous ammonium acetate produced protonated molecular ions for glutathione conjugates of menadione, styrene oxide, pentachlorophenyl methyl sulfone, chlorodinitrobenzene, and chlorambucil. Since, the high percentages of organic modifier needed for good molecular ion intensity preclude chromatographic separation of these polar compounds, successful graphic separation of these polar compounds, successful LC/MS was facilitated by postcolumn addition of organic modifiers to the mobile phase. This new methodology allowed excellent chromatographic separations and thermospray ionization mass spectra to be obtained for a mixture of haloalkane glutathione conjugates. Moreover, cleavage of the gamma-glutamyl-cysteine amide bond of glutathione results in class-characteristic fragment ions. Changes in the fragmentation pathways in spectra acquired with and without organic modifiers shed light on the importance of the desolvation process in obtaining good molecular ion sensitivity in thermospray.

Desorption of ions from rat membranes: selectivity of different ionization techniques

Complex lipid biomarkers, including phosphatidylcholines, cerebrosides and sulfatides, are shown to be desorbed intact from rat brain myelin and rat liver microsomes by liquid secondary ion mass spectrometry, by plasma desorption and by laser desorption. Different polar lipids are favored by the different desorption techniques and as negative or positive ions. These selectivities support current theories about ionization for the different techniques.

Studies of intramolecular rearrangements of acyl-linked glucuronides using salicylic acid, flufenamic acid, and (S)- and (R)-benoxaprofen and confirmation of isomerization in acyl-linked delta 9-11-carboxytetrahydrocannabinol glucuronide

NMR and HPLC have been used to investigate the rearrangements of 1-O-acylglucuronides in vitro and the occurrence of rearranged isomers in urine. Glucuronides of flufenamic acid, (S)- and (R)-benoxaprofen, salicylic acid, and delta 9-11-carboxytetrahydrocannabinol were synthesized, by use of immobilized enzymes, or purified from urine. Ester-linked isomers of these gluruconides were characterized, and isomers derived from flufenamic acid, (S)-benoxaprofen, and salicylic acid were purified for further study by NMR and HPLC. The positions of the new ester linkages could be identified by two-dimensional NMR. Shifts not only in the resonance of the proton adjacent to the esterified hydroxyl group but also in the resonance of the anomeric proton on carbon 1 of the glucuronic acid moiety could be correlated with the position of each isomeric ester bond. HPLC elution times also correlated with ester position in this small set of samples. The sequences of isomer formation were studied in situ by NMR and also at pH 8 by HPLC. These studies indicate that, for the three cases examined, the C-2 ester is formed first, followed by formation of C-3 and C-4 esters. The purified isomeric esters were found not to re-form the high-energy 1-O-acyl bond. All other rearrangement steps are reversible. In contrast to other glycosides and glycerol esters, no evidence could be found for rearrangements beyond nearest-neighbor hydroxyl groups in glucuronic acid. The sequence of formation and reversibility is consistent with an ortho ester intermediate, as has been proposed for rearrangements of other glycosides.(ABSTRACT TRUNCATED AT 250 WORDS)

Solvent-sample interactions in thermospray mass spectrometry of antineoplastic nitrogen mustards

Conditions are reported for the optimization of thermospray mass spectrometric analysis of antineoplastic nitrogen mustard alkylating agents. In aqueous ammonium acetate mobile phase, multiple sequential solvolytic reactions occur with these highly labile compounds, and protonated molecular ions of the reaction products are observed. However, when high proportions of acetonitrile or other organic modifier are added to the mobile phase, solvolytic reactions are much reduced and abundant protonated molecular ions are detected. One exception to these observations is phosphoramide mustard, which solvolyses under all conditions attempted. A lower limit for detection of melphalan using direct injection and summing the ion current between m/z 120 and 870 is about 150 ng. Successful thermospray liquid chromatography/mass spectrometry of these compounds should be possible using high percentages of methanol in the mobile phase or increasing the organic content by post-column solvent modification.

Characterization of novel glycolipids from the giant cockroach (Blaberus colosseus)

A novel class of glycolipids, assigned the trivial name blaberosides, was isolated from whole head tissues of the giant cockroach (Blaberus colosseus). The class consists of two closely related families, blaberoside I and blaberoside II, each containing species differing by 26 atomic mass units. The structure of these gentiobiose-based glycoglycerolipids was elucidated by chromatographic behavior, nuclear magnetic resonance spectroscopy, mass spectrometry, and analysis of chemical degradation products and derivatives. Species in the blaberoside I family have been identified as 2-O-[6'-O-(6"-O-3-hydroxy-11-eicosenoyl-beta-D-glucopyranosyl)-bet a-D- glucopyranosyl]-3-(hexadecyloxy)-1-(3-hydroxy-11-eicosenoyl)-1,2-p ropanediol (blaberoside Ia) and 2-O-[6'-O-(6"-O-3-hydroxy-11-eicosenoyl-beta-D-glucopyranosyl)-bet a- D-glucopyranosyl]-3-(6-octadeceloxy)-1-(3-hydroxy-11-eicosenoyl )-1,2- propanediol (blaberoside Ib). Two smaller homologs of the blaberoside II family were discerned to be 2-O-[6'-O-(6"-O-3-hydroxy-11- eicosenoyl-beta-D-glucopyranosyl)-beta-D-glucopyranosyl]-3-(hex ade cyloxy)- 1,2-propanediol (blaberoside IIa), and 2-O-[6'-O-(6"-O-3-hydroxy-11-eicosenoyl-beta-D- glucopyranosyl)-beta-D-glucopyranosyl]-3-(4-octadeceloxy)-1,2-prop anediol (blaberoside IIb). These compounds are unique because they are animal origin glyceroglycolipids with a highly flexible gentiobiose backbone, and a beta-linkage of the carbohydrate to the glycerol ether at the 2 position rather than the usual 1 position.

2,4-Dinitrophenylhydrazides of polysialogangliosides

Treatment with 2,4-dinitrophenylhydrazine HCl in the presence of dicyclohexylcarbodiimide, converts gangliosides to their dinitrophenylhydrazides. This derivatization is the basis of a useful method for HPLC determination of gangliosides (K. Miyazaki, N. Okamura, Y. Kishimoto and Y. C. Lee (1986) Biochem. J. 235, 755-761). This procedure, however, yields two different GT1b products. By characterizing these two products using plasma desorption mass spectrometry, proton magnetic resonance and other chemical and physical techniques, we found that either one or two of the three sialic acid carboxyl groups in GT1b, were converted to dinitrophenylhydrazides. The remaining underivatized carboxyl groups formed lactones with hydroxyl groups from other carbohydrate residues. Also, while sialic acid residues of GD1a were fully derivatized, only one sialic acid in GD1b, two sialic acids in GT1a and two in GQ1b were converted to dinitrophenylhydrazides, the remaining carboxyl groups probably forming lactones. Sialic acid residues between galactose of the gangliotetraose chain and another sialic acid in polysialogangliosides appear to be underivatized possibly because of steric hindrance.

Characterization of acyl-linked glucuronides by electron impact and fast atom bombardment mass spectrometry

Thirty-one electron impact (EI) mass spectra and 22 fast atom bombardment (FAB) mass spectra were evaluated with regard to providing molecular weights and information about the structures of 1-O-acyl glucuronides. Molecular ion species were obtained by both techniques. Fragmentation of the glycosidic and carboxylate bonds produced ions characteristic of glucuronides as a class, and also ions unique to acyl glucuronides. In EI mass spectra of pertrimethylsilylated derivatives, pairs of [M - 481]+ and [M - 509]+ ions characterized the acyl linkage. Relative abundances within these pairs correlated with the benzylic, benzoic or aliphatic nature of the carboxylate group. Positive ion FAB spectra contained three sets of ions, with intervals of 28 and 46 mass units, which characterized the linkage. In negative ion FAB spectra, a characteristic pair of fragment ions 44 mass units apart were accompanied by anions of mass 193, which appeared to distinguish acyl from phenol and hydroxyl glucuronides.

Use of immobilized enzymes in drug metabolism studies

The immobilization of drug-metabolizing enzymes onto polymeric supports offers several advantages over use of conventional microsomal or soluble enzyme preparations. These include increased storage stability, facilitated separation of products from the incubation mixture, the ability to recover and reuse the enzyme catalyst, and in many cases, stabilization of the tertiary structure of membrane-bound enzymes. Attachment of the protein to the solid support may be accomplished by adsorption, covalent bonding, or entrapment techniques. This methodology has been successfully utilized for studies with such enzymes as cytochrome P-450, UDP-glucuronyltransferases, glutathione S-transferases, S-methyltransferases, and N-acetyltransferases. Although often employed for the synthesis of xenobiotic metabolites, immobilized enzymes have been used for mechanistic and relative reactivity studies, limited kinetic studies, and extracorporeal detoxification. Co-immobilization of multiple drug-metabolizing enzyme systems has made possible the sequential formation of metabolites arising from oxidation followed by conjugation. Immobilized enzymes may also be used in the prediction of species-dependent metabolic pathways. The potential for large-scale synthesis of drug metabolites using this methodology has been explored.

Enzymatic mechanisms of resistance to alkylating agents in tumor cells and normal tissues

The presence in tumor cells and in normal cells of enzymes which metabolize and inactive alkylating agents appears to play a major role in determining the effectiveness of alkylating agents against human tumors and the toxicities of these agents to normal tissues. The enzyme aldehyde dehydrogenase appears to protect bone marrow and the gastrointestinal tract against toxicity from cyclophosphamide and other closely related oxazophosphorine agents. The presence of this enzyme in bone marrow stem cells facilitates the elimination of tumor cells from bone marrow suspensions, with preservation of the ability of the marrow suspension to reconstitute normal hematopoiesis in a patient. A variety of mouse and human tumors has been shown to be resistant to cyclophosphamide on the basis of an elevated aldehyde dehydrogenase content. The clinical significance of this type of resistance is currently being explored. Increased levels of glutathione-S-transferase have been shown to be associated with cellular resistance to a variety of alkylating agents. We have identified and characterized the conjugates of nitrogen mustards with glutathione. The formation of these conjugates is catalyzed by glutathione-S-transferase. The further study and characterization of these specific reactions should contribute to the understanding and quantitation of this type of alkylating agent resistance.

Species-dependent enantioselective glucuronidation of three 2-arylpropionic acids. Naproxen, ibuprofen, and benoxaprofen

The enantioselective glucuronidation of several racemic 2-arylproprionic acids (naproxen, ibuprofen, and benoxaprofen) was investigated in vitro with immobilized microsomal protein from human, rhesus monkey, and rabbit liver as the source of UDP-glucuronyl-transferases. Human microsomes, solubilized microsomal protein, and immobilized protein all gave comparable enantioselectivity. The diastereomeric glucuronides were separated and quantitated by HPLC and characterized stereochemically by co-elution with glucuronides formed from authentic resolved enantiomers. Conjugation of the carboxylic acid moieties occurred stereoselectively with all three substrates. However, enantioselectivity varied qualitatively and quantitatively with substrate as well as with species. The glucuronidation of (S)-naproxen by human liver enzymes was inhibited in the presence of (R)-naproxen and vice versa. The ratio of the glucuronides of (S)-benoxaprofen to that of (R)-benoxaprofen in rhesus monkey urine varied between individual animals and appeared to change through time as dosing continued. Hydrolysis of the diasteriomeric glucuronides of (R)- and (S)-naproxen was differentially inhibited by addition of 1,4-saccharolactone.

Phosphorylation heterogeneity of tryptic phosphopeptides of chicken riboflavin-binding protein

The tryptic phosphopeptide of hen egg white riboflavin-binding protein has been found to exist as a mixture of peptides which differ only with respect to the number of covalently bound phosphoryl groups. Anion-exchange chromatography was used to separate homologues of the tryptic phosphopeptide of egg white riboflavin-binding protein. Four peptide peaks were obtained and analyzed using plasma desorption mass spectrometry. Molecular ions obtained agree closely with calculated molecular weight values for phosphopeptides with 8, 7 and 5 phosphoryl groups. Amino acid analyses showed that the octa- and hepta-phosphorylated peptides were pure and had the same amino acid compositions.

Non-enzymatic formation of insulin-glutathione mixed disulfides: evidence for a transient species by plasma desorption mass spectrometry

Formation of insulin-glutathione mixed disulfides takes place under the conditions of 0.1 M ammonium acetate, neutral pH and without the presence of any enzyme. Using a SH-free glutathione-agarose column it is demonstrated that the interaction of insulin with glutathione is specific, and increasing the incubation time between these two peptides results in the reduction of insulin disulfide bonds and the production of A and B chains.

Species-dependent glucuronidation of drugs by immobilized rabbit, rhesus monkey, and human UDP-glucuronyltransferases

The utility of immobilized microsomal enzymes from rabbit liver for the synthesis of a variety of conjugated drug metabolites has been demonstrated in our laboratory. Here, this capability is extended to microsomal preparations from monkey and human liver. A comparative study of glucuronidation of various substrates was carried out using immobilized uridine-5'-diphosphoglucuronyltransferase from rabbit, rhesus monkey, and human liver. The three drugs used for the study (sulfadimethoxine, oxazepam, and cyproheptadine) were chosen on the basis of their previously reported in vivo species differences in glucuronidation. Aglycons were incubated with UDP-glucuronic acid in the presence of immobilized UDP-glucuronyl-transferase at pH 7.4 for 16 hr at 37 degrees C. Glucuronide products were purified by high performance liquid chromatography and further characterized by thin layer chromatography and mass spectometry. Species-dependent glucuronidation was observed in all three cases. Differences in the in vitro product formation paralleled in vivo species differences for the three drugs studied. The same glucuronides were produced by immobilized human liver microsomes as have been found to be formed in vivo from the three drugs studied.

Conversion of melphalan to 4-(glutathionyl)phenylalanine. A novel mechanism for conjugation by glutathione-S-transferases

One of the conjugates of melphalan, characterized following incubation with glutathione (GSH) and immobilized microsomal glutathione-S-transferases, has been identified as 4-(glutathionyl)-phenylalanine. This conjugate is formed by displacement of the mustard moiety. The structure was confirmed by reaction of the corresponding 4-halophenylalanines with GSH as well as by TLC, HPLC, and FAB mass spectrometry. Evidence is presented here to support the hypothesis that this novel reaction occurs via a cyclic aziridinium ion. To test this proposed mechanism, N,N-dimethyl-p-toluidine and its corresponding quaternary ammonium iodide salt were incubated with GSH in the presence of immobilized glutathione-S-transferases at 37 degrees C for 1 hr at pH 7.4. The tertiary amine did not react, whereas the quaternary compound produced 4-(glutathionyl)toluene. The effect of ring substituent requirements for the reaction was evaluated. The formation of GSH adducts of alkylating agents may be a factor in the development of resistance to these drugs.

Mass spectral analysis of complex lipids desorbed directly from lyophilized membranes and cells

Three desorption ionization techniques--laser desorption, plasma desorption and fast atom bombardment mass spectrometry--have been applied to lyophilized cells, membranes, lysed cells and various extracts. It has been shown that intact polar lipids are selectively desorbed from biological membranes by these methods and that their mass spectra provide "fingerprints" which reflect the unique biochemical composition of each class of cell or membrane.

Glutathione conjugates. Immobilized enzyme synthesis and characterization by fast atom bombardment mass spectrometry

Glutathione transferase activity was shown to be present in an immobilized preparation of microsomal protein. Chlorodinitrobenzene, ethacrynic acid, captopril, styrene oxide, and iminocyclophosphamide were found to be substrates, each providing a different kind of electrophilic functional group for conjugation. The glutathione conjugates were characterized by thin layer chromatography (visualized by reaction with ninhydrin) and by high pressure liquid chromatography. A variety of conditions was evaluated for analysis of these glutathiones by fast atom bombardment mass spectrometry.