Phosphorylation sites in riboflavin-binding protein characterized by fast atom bombardment mass spectrometry

The capability of fast atom bombardment mass spectrometry for characterization of phosphorylation sites in a tryptic peptide from chicken egg yolk riboflavin-binding protein has been evaluated. The quality of information about molecular weight, amino acid sequence, phosphorylation sites, and microheterogeneity is evaluated as a function of the sign of the ions analyzed, the nature of the counter ions associated with the phosphate substituents, sample matrix, and various instrumental parameters. The intact octaphosphorylated 23-residue peptide was found to be susceptible to mass spectral analysis. Information from the negative ion spectrum was used in conjunction with complete sequence information and experiments which showed that all phosphates were attached to serine residues. Phosphorylated and unphosphorylated serine residues were identified and the sample was shown to be homogeneously octaphosphorylated.

Mass spectral analysis of murine epidermal growth factor

Fast atom bombardment mass spectrometry has been used to characterize epidermal growth factor isolated from mouse submaxillary glands. The preparation is found to consist of two peptides, one of which has the average molecular weight predicted for the familiar gene product. The molecular weight of the second component is found to be reduced by the mass of one asparagine residue. These observations are discussed in light of previous reports of heterogeneity.

Acylation of albumin by 1-O-acyl glucuronides

Representative examples of drug metabolites containing a carboxylic acid group conjugated to glucuronic acid are shown to be active chemical electrophiles, which acylate albumin in vitro through transesterification reactions. Based on these and other observations, we propose that this acylating reactivity is characteristic of 1-O-acyl glucuronides as a class, and that albumin is representative of many susceptible biopolymers. The hypothesis is advanced that this reaction can occur in vivo as well, and that it may offer a molecular mechanism for the physiological activity or toxicity of some xenobiotic and lipophilic compounds.

Position of ester groups in the lipid A backbone of lipopolysaccharides obtained from Salmonella typhimurium

Lipopolysaccharides extracted from the heptoseless mutant of Salmonella typhimurium G30/C21 were hydrolyzed with either 0.1 N HCl at 100 degrees C or treated twice with 20 mM sodium acetate, pH 4.5, at 100 degrees C for 45 min and finally purified by preparative thin layer chromatography to yield a structural series of mono- and diphosphoryl lipid A, respectively. Positive ion fast atom bombardment mass spectrometry of the diphosphoryl lipid A TLC-3 (a highly acylated major band) showed a major component with (M + H)+ ion of mass 1798, which fragmented to yield a (M - H2PO4)+ ion of mass 1700. Cleavage at the glycosidic bond gave rise to an oxonium ion fragment of mass 1087. In conjunction with other studies, this establishes the molecular formula and Mr of the major component to be C94H178N2O25P2 and 1797.2 (as the free acid), respectively. Similar analysis of monophosphoryl lipid A TLC-3 produced an (M + H)+ peak at m/z 1718, (M + Na)+ adduct peak at m/z 1740, and a fragment of mass 1087. The spectrum of monophosphoryl lipid A TLC-5 was devoid of the m/z 1087 peak and instead contained the phosphorylated oxonium ion of mass 876. This fragment ion is assigned as the distal subunit, and these results show that the distal subunit of the major lipid A TLC-3 contains two hydroxymyristoyl, one myristoyl, and one lauroyl residues, whereas the reducing end subunit contains two hydroxymyristoyl groups. A revised structure of the lipid A backbone in lipopolysaccharides of S. typhimurium is proposed.

Dipalmitoylphosphatidylcholine in amniotic fluid quantified by fast-atom-bombardment mass spectrometry

Dipalmitoylphosphatidylcholine (DPC) is quantified by taking advantage of stable-isotope-labeled d9-DPC as internal standard. Use of a mass spectrometer to measure the ratio of d9/d0 makes this procedure a quantitative one. d9-DPC was synthesized by refluxing dipalmitoylethanolamine with d3-methyl iodide in methanol in the presence of sodium bicarbonate for 26 h. The yield of d9-phosphatidylcholine (d9-lecithin) was 89% after column-chromatographic purification. Fast atom bombardment was used to desorb the preformed phosphatidylcholine ions in a mass spectrometer of Nier-Johnson geometry. In our assessment of accuracy and precision of this technique, we found a correlation coefficient of 0.9994 between signal and sample concentration. The method was less precise when the total d0- plus d9-DPC was less than 0.2 micrograms or when the ratio of d0- to d9-lecithin exceeded 100. The within-run CV was about 1.0%. The amount of DPC in amniotic fluid samples assessed by mass spectrometry was compared with results for total phosphatidylcholine quantified by thin-layer chromatography. The fate of DPC in various laboratory manipulations was also studied.

Dipalmitoylphosphatidylcholine in amniotic fluid quantified by fast-atom-bombardment mass spectrometry

Dipalmitoylphosphatidylcholine (DPC) is quantified by taking advantage of stable-isotope-labeled d9-DPC as internal standard. Use of a mass spectrometer to measure the ratio of d9/d0 makes this procedure a quantitative one. d9-DPC was synthesized by refluxing dipalmitoylethanolamine with d3-methyl iodide in methanol in the presence of sodium bicarbonate for 26 h. The yield of d9-phosphatidylcholine (d9-lecithin) was 89% after column-chromatographic purification. Fast atom bombardment was used to desorb the preformed phosphatidylcholine ions in a mass spectrometer of Nier-Johnson geometry. In our assessment of accuracy and precision of this technique, we found a correlation coefficient of 0.9994 between signal and sample concentration. The method was less precise when the total d0- plus d9-DPC was less than 0.2 micrograms or when the ratio of d0- to d9-lecithin exceeded 100. The within-run CV was about 1.0%. The amount of DPC in amniotic fluid samples assessed by mass spectrometry was compared with results for total phosphatidylcholine quantified by thin-layer chromatography. The fate of DPC in various laboratory manipulations was also studied.

Quaternary ammonium-linked glucuronides of amitriptyline, imipramine, and chlorpromazine

The quaternary ammonium-linked glucuronides of amitriptyline, imipramine, and chlorpromazine were shown to be conjugated in vitro using an immobilized rabbit hepatic microsomal enzyme preparation. It was shown that fast atom bombardment mass spectrometry could be used for direct characterization of these involatile, thermally labile metabolites. The quaternary ammonium-linked glucuronides of amitriptyline and imipramine were demonstrated to be present in urine from patients receiving therapeutic doses of these tricyclic antidepressants.

Synthesis and analysis of glucuronides of N-hydroxyamides

Reactions of five N-hydroxyacetylarylamines (N-hydroxyphenacetin, N-hydroxyacetanilide, N-hydroxy-p-chloroacetanilide, N-hydroxy-2-acetylaminonaphthalene, and N-hydroxy-2-acetylaminofluorene) with uridine 5'-diphosphoglucuronic acid catalyzed by immobilized glucuronyl transferase enzyme resulted in the formation of glucuronides conjugated at the N-hydroxyl group. Chromatographic properties of these compounds were examined by using thin layer, gas liquid, and high pressure liquid chromatography. A variety of mass spectrometric techniques were also evaluated for characterization of compounds of this class, including fast atom bombardment, chemical ionization, and electron impact. Electron impact spectra of the trimethylsilyl derivatives contain class characteristic ions and fragmentation pathways that would permit the special nature of the site of conjugation of this important class of compounds to be recognized in glucuronides of undetermined structure. Molecular ion species were not reliably detected in the electron impact spectra; however, these were readily distinguishable by the other two techniques.

Purification and characterization of the heat-stable factors essential for the conversion of lignoceric acid to cerebronic acid and glutamic acid: identification of N-acetyl-L-aspartic acid

The conversion of lignoceric acid to cerebronic acid, ceramides, cerebrosides, and glutamic acid is catalyzed by a rat brain particulate preparation. The heat-stable factor, prepared from calf cerebellum, together with the heat-labile factor, a pyridine nucleotide, and Mg2+ are essential to all of these metabolic pathways. Our previous work showed that the heat-stable factor is composed of at least two components, HSF-1 and HSF-2, and identified HSF-2 as D-glucose-6-phosphate. In the current investigation, HSF-1 was further purified and found to be N-acetyl-L-aspartic acid. In addition, it was discovered that a third component, HSF-3, is also required for heat-stable factor activity. A reconstituted system composed of N-acetylaspartic acid, glucose-6-phosphate, and HSF-3 fully replaced the heat-stable factor essential for the conversion of lignoceric acid to cerebronic acid and glutamic acid. The reconstituted heat-stable factor did not show the initial time lag always observed with the crude heat-stable factor.

Iminocyclophosphamide as a chemically reactive metabolite of cyclophosphamide

Iminocyclophosphamide has been identified among the metabolites produced by incubation of cyclophosphamide with cytochrome P-450 mono-oxygenase immobilized on beaded Sepharose. The reactive imine was trapped by addition of hydrogen cyanide, and the product, 4-cyanocyclophosphamide, was identified by comparison of its chromatographic and mass-spectral properties with those of a synthetic standard. Iminocyclophosphamide was also generated chemically from 4-hydroperoxycyclophosphamide and characterized both by addition of hydrogen cyanide and by reduction with sodium borodeuteride. The synthesis of authentic 4-cyanocyclophosphamide is also reported.

Metabolic formation of N- and O-glucuronides of 3-(p-chlorophenyl)thiazolo[3,2-a]benzimidazole-2-acetic acid. Rearrangement of the 1-o-acyl glucuronide

Excretion of 3-(p-chlorophenyl)thiazolo[3,2-a]benzimidazole-2-acetic acid (I) and its metabolites was studied in rats, beagle dogs, and rhesus monkeys given 20-mg/kg doses of 14C-labeled drug. The urine of rhesus monkeys contained two metabolites in addition to unchanged drug. Both metabolites were hydrolyzed to I by beta-glucuronidase and the hydrolysis was inhibited by 1,4-saccharolactone, indicating that they were glucuronides of I. One of the metabolites (III) was not hydrolyzed by dilute alkali. Its NMR spectrum indicated that the site of conjugation was one of the nitrogen atoms, i.e., it was a quaternary N-glucuronide. The FAB mass spectrum was in conformity with this assignment. This metabolite was not present in the urine of dogs or rats given labeled drug. The other metabolite (II) was excreted in the urine of all three species as well as in the bile of the rat. It was readily hydrolyzed by dilute alkali (pH 11 for 0.5 hr at 37 degrees C), indicating that this metabolite was an acyl glucuronide. The metabolite was stable at pH 4.5 but it was readily converted to three isomers at 37 degrees C within 1 hr at pH 6.5 and above. The mass spectra of the derivatized isomers and metabolite were similar. The isomers were hydrolyzed to I by dilute alkali but not by beta-glucuronidase. They exhibited reducing properties (whereas metabolite II did not), suggesting that they were formed by acyl migration of the aglycone to the second, third, and fourth carbon atoms of the glucuronic acid moiety. Acyl migration probably plays a role in the disposition of I as well as other drugs that form labile glucuronides.

Electrophilic reactions of acyl-linked glucuronides. Formation of clofibrate mercapturate in humans

An acyl-linked mercapturic acid metabolite of clofibrate has been identified in human urine. The identification is based on comparison of fast-atom bombardment mass spectra and high-pressure liquid chromatograms with those of synthesized material. This represents the first acyl-linked mercapturate found in man. The mechanism proposed for its formation involves transacylation of clofibrate acyl glucuronide by glutathione, hydrolysis, and acetylation to produce clofibryl mercapturic acid. Synthetic clofibrate glucuronide is shown to be electrophilic; it reacts covalently with ethanethiol and p-nitro-benzylpyridine. We propose that clofibrate acyl glucuronide is an electrophilic metabolite which reacts with sulfhydryl groups and therefore may be responsible for the human hepatotoxicity of clofibrate.

Synthesis of quaternary ammonium-linked glucuronides by rabbit hepatic microsomal UDP-glucuronyltransferase and analysis by fast-atom bombardment mass spectrometry

The quaternary ammonium-linked glucuronides of tripelennamine and cyproheptadine, important human metabolites of these drugs, were synthesized with an immobilized rabbit hepatic microsomal enzyme system as catalyst. The glucuronide of pyridine was prepared by chemical synthesis. These cationic, thermally labile, quaternary ammonium-linked glucuronides were analyzed by fast-atom bombardment mass spectrometry. The mass spectra contained intense molecular cations and characteristic losses of neutral fragments. The information contained in the spectra was sufficient to characterize the difficult-to-analyze intact quaternary ammonium glucuronides and provide structural information about the aglycon.

Stable isotope analysis by fast-atom bombardment labeling of UDP-glucuronic acid

Fast-atom bombardment mass spectrometry is found to provide a method for analysis of isotopes in the enzyme cofactor uridine-5'-diphosphoglucuronic acid, heretofore unsusceptible to mass-spectral characterization. This technique was used to determine optimal conditions for the introduction of 18O by acid-catalyzed exchange in H218O and to evaluate the loss of the isotope when labeled cofactor is used in enzymatic incubations. Fast-atom bombardment mass spectrometry provided a quantitative assessment of various isotopic species and also permitted the location of the isotopes in the molecule to be determined.

Comparison of biological-derived and synthetic leukotriene C4 by fast atom bombardment mass spectrometry

The development of the technique of fast atom bombardment mass spectrometry (FAB-MS) has greatly expanded the capability to analyze non-volatile, complex biochemicals. The structure of leukotriene C4, a slow reacting substance of anaphylaxis, has recently been postulated as 6-glutathionyl-5-hydroxy-7,9,11,14-eicosatetraenoic acid. Even though LTC4 has been synthesized, it has not been possible to obtain direct mass spectrometric confirmation of this structure. FAB-MS has indicated that the biological LTC4 had a molecular weight of 625, identical to that of synthetic LTC4. The abundance of cationized species with one and two sodium atoms was the major difference between these leukotrienes which is probably a result of the difference in salt content of the purified molecules.

Simultaneous immobilization of cytochrome P-450 and glucuronyltransferase for synthesis of drug metabolites

Cytochrome P-450, NADPH-cytochrome P-450 reductase, and glucuronyltransferase were immobilized simultaneously on cyanogen bromide-activated Sepharose from phenobarbital-induced rabbit liver microsomes. The activity of the P-450 system was demonstrated by the N-demethylation of ethylmorphine and the O-demethylation of p-nitroanisole. p-Nitrophenol produced from the oxidation of p-nitroanisole was conjugated to p-nitrophenyl glucuronide, as evidenced by isolation and characterization of the glucuronide by GC/MS.

Quantitation by gas chromatography-chemical ionization-mass spectrometry of phenylalanine mustard in plasma of patients

An unambiguous and sensitive method based on gas chromatography-chemical ionization-mass spectrometry has been developed to quantitate L-phenylalanine mustard and has been applied to measure levels in plasma of five patients receiving 0.15 to 0.25 mg/kg (10 to 17 mg) of the drug p.o. Peak plasma levels of 50 to 190 ng/ml were found to occur between 0.7 and 2.3 hr after ingestion. The time for the plasma level to fall to one-half of the peak value varied from 0.6 to 3 hr, and very low levels (less than 2 ng/ml) were present by 24 hr.

The effects of heating rate and sample size on the direct exposure/chemical ionization mass spectra of some biological conjugates

A direct exposure probe has been used to obtain mass spectra of underivatized guanosine, deoxyguanosine, sucrose and the p-nitrophenyl-beta-D-glucuronide. In all cases, a protonated molecular ion is produced with good relative abundance. The effects of heating rate and sample size on the production of the [MH]+ ION ARE EXAMINED IN DETAIL FROM TOTAL ION ANd single ion currents produced during rapid, repetitive scanning of the spectra after probe insertion. From this data we conclude that protonated molecular ions are produced as a result of the enhanced volatility of neutral molecules on the probe surface, followed by chemical ionization, and not by surface ionization.

Enzymatic conjugation and hydrolysis of [18O]isoborneol glucuronide

Introduction of 18O into the acetal linkage of isoborneol glucuronide has been accomplished through the in vitro enzymatic conjugation of 18O-labeled isoborneol by use of immobilized, partially purified, rabbit liver UDP-glucuronyltransferase supplemented with UDP-glucuronic acid. Mass-spectrometric analysis of 18O enrichment in both the aglycon and the intact conjugate made it possible to study both enzymatic conjugation and enzymatic hydrolysis. Analysis by gas chromatography-mass spectrometry indicated that the labeled hydroxyl oxygen of isoborneol is retained during both conjugation and hydrolysis.

Synthesis and characterization of glucuronides of 5'-hydroxy-delta9-tetrahydrocannabinol and 11-hydroxy-delta9-tetrahydrocannabinol

Glucuronides of two metabolites of delta9-tetrahydrocannabinol, 11-hydroxy-delta9-THC and 5'-hydroxy-delta9-THC, have been synthesized and characterized. Two isomeric monoglucuronides were isolated from the incubation of each metabolite with UDP-glucuronyltransferase immobilized on beaded Sepharose. The structures of these conjugates were assigned primarily on the basis of combined gas chromatography-mass spectrometry.

Quantitation by gas chromatography-chemical ionization mass spectrometry of cyclophosphamide, phosphoramide mustard, and nornitrogen mustard in the plasma and urine of patients receiving cyclophosphamide therapy

Unambiguous and sensitive methods based on gas chromatography-chemical ionization mass spectrometry have been developed to quantitate cyclophosphamide and two alkylating and cytotoxic metabolites, phosphoramide mustard and nornitrogen mustard. The levels of these materials have been determined in the plasma and urine of five patients receiving cyclophosphamide, 60 or 75 mg/kg i.v. Peak plasma levels of phosphoramide mustard of 50 to 100 nmoles/ml were found at 3 hr after cyclophosphamide administration. Variable levels of nornitrogen mustard were found in the plasma. This product may be arising in part from the decomposition of other metabolites during sample storage and preparation.

Mandelonitrile beta-glucuronide: synthesis and characterization

Mandelonitrile beta-glucuronide, the compound patented as Laetrile, has been synthesized from rabbit liver uridine diphosphate-glucuronosyl transferase immobilized on beaded sepharose, has been analyzed by thin-layer chromatography, nuclear magnetic resonance, and gas chromatography-mass spectrometry, and has been tested for cytotoxicity and mutagenic activity with Salmonella typhimurium strains TA 98 and TA 100. Several commercial laetrile preparations contained no glucuronide; they contained amygdalin and neoamygdalin instead. Mandelonitrile, mandelonitrile glucuronide, and a mixture of amygdalin and neoamygdalin were each found to be mutagenic.

Identification of aldophosphamide as a metabolite of cyclophosphamide in vitro and in vivo in humans

Aldophosphamide (NSC 254), a putative key metabolite of cyclophosphamide, has now been isolated as a cyanohydrin derivative from an incubation mixture of cyclophosphamide with mouse liver microsomes in vitro and from the plasma of a cyclophosphamide-treated patient. The cyanohydrin has been shown to be identical with an authenic synthetic sample by mass spectrometry and combined gas chromatography-mass spectrometry.

Synthesis and characterization of glucuronides of Cannabinol, cannabidiol, delta9-tetrahydrocannabinol and delta8-tetrahydrocannabinol

Partially purified glucuronyltransferase immobilized on beaded sepharose has been used to synthesize the glucuronide conjugates of cannabinol, cannabidol, delta9-tetrahydrocannabinol and delta8-tetrahydrocannabinol. Trimethylsilylated methyl esters and per(trimethylsilyl) derivatives of these conjugates have been characterized by their gas chromatographic retention times and their electron impact and ammonia chemical ionization mass spectra.

Review of the metabolism and mode of action of cyclophosphamide

A review of recent studies of the pharmacology of cyclophosphamide is presented. Particular attention is paid to the use of mass spectrometry in the identification of the active compound phosphoramide mustard as a circulating human metabolite of cyclophosphamide, the development of an assay method for cyclophosphamide, phosphoramide mustard, and nornitrogen mustard, and a demonstration that alkylation by phosphoramide mustard proceeds through a symmetrical aziridinium ion.

Solid-phase synthesis of drug glucuronides by immobilized glucuronosyltransferase

Rabbit liver glucuronosyltransferase immobilized on beaded agarose has been used to synthesize glucuronic acid conjugates of meprobamate, diethylstibestrol, bilirubin, borneol, benzioc acid, and p-nitrothiophenol. The immobilized enzyme exhibited a high degree of specificity for UDPGA as cofactor when p-nitrophenol is used as substrate. Other cofactors tested were less effective, all producing less than 10% conjugation relative to UDPGA. The effects on agarose-bound enzyme activity of a variety of cosolvents and emulsifiers have been studied. Ethanol, dimethyl sulfoxide, propylene glycol, and bovine serum albumin are among the cosolvents and emulsifiers which can be used within limited concentration ranges to sulubilize lipophilic substrates for conjugation. Concentrations of calcium and magnesium cations between 1.5 and 10.0mM were found to enhance glucuronosyltransferase activity of the immobilized enzyme.

Approaches to the pharmacokinetics of cyclophosphamide (NSC 26271): Quantitation of metabolites

Recent evidence has implicated phosphoramide mustard as a highly active metabolite of cyclophosphamide (CP). This compound has been shown to be produced from CP both in vitro and in vivo. To further study the role of this active metabolite we have devised methods to quantitate the compound in human serum and urine using gas chromatography-chemical ionization mass spectrometry. Tetradeuterated phosphoramide mustard has been synthesized and is used as an internal standard. CP and nor-nitrogen mustard have also been quantitated in human samples using analogous tetradeuterated internal standards.

Alkylating properties of phosphoramide mustard

The relative alkylating activities of two of the cytotoxic metabolites of cyclophosphamide, phosphoramide mustard and nornitrogen mustard, have been studied at pH 4.6 and 7.4. The products formed on alkylation of ethanethiol by these metabolites have been identified, confirming that phosphoramide mustard undergoes alkylation reactions as an intact molecule. Deuterated analogs of the two metabolites have been synthesized, namely N,N-bis(2,2-dideutero-2-chloroethyl)-phosphorodiamidic acid and N,N-bis(2,2-dideutero-2-chloroethyl)amine and used to determine that alkylation proceeds directly via an aziridinium intermediate rather than a direct SN2 displacement of the chlorine atom.

Immobilized glucuronosyltransferase for the synthesis of conjugates

Partially purified rabbit liver UDPglucuronosyltransferase is immobilized on agarose by the cyanogen bromide activation method. Both soluble and matrix-bound enzyme preparations display very similar Km and pH optimum. The storage stability of the immobilized enzyme at 4 degrees is 5-10 times improved over the soluble preparations. The agarose-bound UDPglucuronosyltransferase is successfully used in the synthesis of p-nitrophenyl glucuronide in an overall yield of 50-70%. The matrix-bound enzyme is reusable over an extended period of time and offers an easy and convenient synthetic tool for various drug glucuronides.

Reaction of 1,3-bis(2-chloroethyl)-1-nitrosourea with synthetic polynucleotides

The antitumor agent BCNU (1,3-bis(2-chloroethyl)-1-nitrosourea) was incubated with poly(C) and poly(G) in aqueous solution at 37 degrees and pH 7 to produce approximately 0.33 and 0.07% substitution, respectively. Under the same conditions, there was relatively little reaction with poly(A) and poly(U). Poylnucleotides reacted with [14C]BCNU were digested by chemical and enzymatic methods, and the derivative nucleotides were isolated by column chromatography. There were identified by a combination of ultraviolet and mass spectroscopy as 3-(beta-hydroxyethyl)CMP, 3,N4-ethano-CMP, and 7-(beta-hydroxy-ethyl)GMP. This would indicate that BCNU generates active two carbon fragments, probably chloroethyl carbonium ions, which are free to react with nucleotides. The production of these substituted bases may be important to the mechanism of action of the therapeutic nitrosoureas since they would probably alter the function of any nucleic acid which contained them.

Identification of phosphorodiamidic acid mustard as a human metabolite of cyclop hosphamide

An active metabolite of cyclophosphamide, N,N-bis(2-chloroethyl)phosphorodiamidic acid, has been confirmed as a circulating and excreted metabolite in patients receiving the drug in therapy, by selected ion monitoring on a gas chromatograph mass spectrometer.