Determination of hydroxyalkyl derivatives of cobalamin (vitamin B12) using reversed phase high performance liquid chromatography with electrospray tandem mass spectrometry and ultraviolet diode array detection

The effect of vitamin B12 on DNA adduction by styrene oxide, a genotoxic xenobiotic

Vitamin B12 (cyano- or hydroxo-cobalamin) acts, via its coenzymes, methyl- and adenosyl-cobalamin, as a partner for enzymatic reactions in humans catalysed by methionine synthase and methylmalonyl-CoA mutase. As well as its association with pernicious anaemia, human B12 deficiency may also be a risk factor for neurological illnesses, heart disease and cancer. In the present work the effect of vitamin B12 (hydroxocobalamin) on the formation of DNA adducts by the epoxide phenyloxirane (styrene oxide), a genotoxic metabolite of phenylethene (styrene), has been studied using an in vitro model system. Styrene was converted to its major metabolite styrene oxide as a mixture of enantiomers using a microsomal fraction from the livers of Sprague-Dawley rats with concomitant inhibition of epoxide hydrolase. However, microsomal oxidation of styrene in the presence of vitamin B12 gave diastereoisomeric 2-hydroxy-2-phenylcobalamins. The quantitative formation of styrene oxide-DNA adducts was investigated using 2-deoxyguanosine or calf thymus DNA in the presence or absence of vitamin B12. Microsomal incubations containing either deoxyguanosine or DNA in the absence of vitamin B12 gave 2-amino-7-(2-hydroxy-1-phenylethyl)-1,7-dihydro-6H-purin-6-one [N7-(2-hydroxy-1-phenylethyl)-guanine], and 2-amino-7-(2-hydroxy-2-phenylethyl)-1,7-dihydro-6H-purin-6-one [N7-(2-hydroxy-2-phenylethyl)guanine] as the principal adducts. With deoxyguanosine the level of formation of guanine adducts was ca. 150 adducts/106 unmodified nucleoside. With DNA the adduct level was 36 pmol/mg DNA (ca. 1 adduct/0.83 × 105 nucleotides). Styrene oxide adducts from deoxyguanosine or DNA were not detected in microsomal incubations of styrene in the presence of vitamin B12. These results suggest that vitamin B12 could protect DNA against genotoxicity due to styrene oxide and other xenobiotic metabolites. However, this potential defence mechanism requires that the 2-hydroxyalkylcobalamins derived from epoxides are not 'anti-vitamins' and ideally liberate, and therefore, recycle vitamin B12. Otherwise, depletion of vitamin B12 leading to human deficiency could increase the risk of carcinogenesis initiated by genotoxic epoxides.

DNA as an in vitro trapping agent for detection of bulky genotoxic metabolites

The instability of electrophilic reactive metabolites in in vitro metabolism studies makes their accurate analysis challenging. To stabilise the reactive compounds prior to their analysis, different trapping agents, such as thiols, amines and cob(I)alamin, have earlier been tested depending on the metabolites to be analysed and the type of study. In the present work, DNA is introduced as a trapping agent for measuring the formation of bulky electrophilic metabolites. Benzo[a]pyrene (B[a]P), a polycyclic aromatic hydrocarbon (PAH), was used as a model compound in a rat liver S9 metabolic system. Under physiological incubation conditions, B[a]P metabolises to diol epoxide (BPDE) metabolites which were trapped by DNA resulting in the formation of covalently bound DNA adducts. The methodology for analysis of these adducts included extraction of the DNA from the metabolic system, digestion of the DNA to yield nucleosides and analysis of the BPDE-adduct to deoxyguanosine (BPDE-dG) by liquid chromatography coupled to high resolution mass spectrometry (HRMS). The chromatographic conditions in combination with the high mass accuracy data (±3 ppm) was useful in resolving BPDE-dG in its protonated form from the complex set of ions present in the metabolic matrix. The method was validated in terms of sensitivity, specificity, accuracy, precision and recovery, and applied to provide a preliminary estimate of BPDE-dG levels from the metabolism of B[a]P in rat S9. The use of DNA as a trapping agent for in vitro metabolites has a potential to aid in cancer risk assessment procedure of PAHs, for instance, in inter-species comparison of metabolism to reactive metabolites and can be adapted for screening of genotoxic metabolites, e.g., from emerging environmental contaminants.

Structural analysis of organometallic compounds with soft ionization mass spectrometry

The analysis of organometallic compounds with mass spectrometry has some special features in comparison with organic and bioorganic compounds. The first step is the choice of a suitable ionization technique, where the electrospray ionization is certainly the best possibility for most classes of organometallic compounds and metal complexes. Some ionization mechanisms of organometallic compounds are comparable to organic molecules, such as protonation/deprotonation, and adduct formation with sodium or potassium ions; however, in many cases, different mechanisms and their combinations complicate the spectra interpretation. Organometallics frequently undergo various types of adduct and polymerization reactions that result in significantly higher masses observed in the spectra in comparison to molecular weights of studied compounds. Metal elements typically have more natural isotopes than common organic elements, which cause characteristic wide distributions of isotopic peaks; for example, tin has ten natural isotopes. The isotopic pattern can be used for the identification of the type and number of metal elements in particular ions. The ionization and fragmentation behavior also depend on the type of metal atom; therefore, our discussion of mass spectra interpretation is divided according to the different type of organometallic compounds. Among various types of mass spectrometers available on the market, trap-based analyzers (linear or spherical ion-traps, Orbitrap) are suitable to study complex fragmentation pathways of organometallic ions and their adducts, whereas high-resolution and high-mass accuracy analyzers (time-of-flight-based analyzers, or Fourier transform-based analyzers-Orbitrap or ion cyclotron resonance mass spectrometers) provide accurate masses applicable for the determination of the elemental composition of individual ions.

Cob(I)alamin for trapping butadiene epoxides in metabolism with rat S9 and for determining associated kinetic parameters

The reduced state of vitamin B(12), cob(I)alamin, acts as a supernucleophile that reacts ca. 10(5) times faster than standard nucleophiles, for example, thiols. Methods have been developed for trapping electrophilically reactive compounds by exploiting this property of cob(I)alamin. 1,3-Butadiene (BD) has recently been classified as a group 1 human carcinogen by the International Agency for Research on Cancer (IARC). The carcinogenicity of BD is considered to be dependent on the activation or deactivation of the reactive metabolites of BD, that is, the epoxides (oxiranes) 1,2-epoxy-3-butene (EB), 1,2:3,4-diepoxybutane (DEB), and 1,2-epoxy-3,4-butanediol (EBdiol). Cytochrome P450 (P450) isozymes are involved in oxidation of BD to EB and further activation to DEB. EB and DEB are hydrolyzed by epoxide hydrolases (EH) to 3,4-dihydroxy-1-butene (BDdiol) and EBdiol, respectively. EBdiol can also be formed by oxidation of BDdiol. In the present study, cob(I)alamin was used for instant trapping of the BD epoxide metabolites generated in in vitro metabolism to study enzyme kinetics. The substrates EB, DEB, and BDdiol were incubated with rat S9 liver fraction, and apparent K(m) and apparent V(max), were determined. The ratio of conversion of EB to DEB (by P450) to the rate of deactivation of DEB by EH was 1.09. Formation of EBdiol from hydrolysis of DEB was ca. 10 times faster than that from oxidation of BDdiol. It was also found that the oxidation of EB to DEB was much faster than that of BDdiol to EBdiol. The study offers comparative enzyme kinetic data of different BD metabolic steps, which is useful for quantitative interspecies comparison. Furthermore, a new application of cob(I)alamin was demonstrated for the measurement of enzyme kinetics of compounds that form electophilically reactive metabolites.

Identification and quantitation of cobalamin and cobalamin analogues in human feces

BACKGROUND

Cobalamin (vitamin B-12) and cobalamin analogues are present in human feces, but a complete identification has not been established, and the amounts present have not been determined.

OBJECTIVES

We aimed to develop a liquid chromatography-mass spectrometry method for cobalamin and cobalamin analogues and to identify and quantitiate the amounts present in human feces.

DESIGN

Fecal samples were obtained from 20 human subjects in good general health. The samples were analyzed for the presence and amounts of cobalamin and 12 cobalamin analogues that were synthesized with and without the incorporation of stable isotopes.

RESULTS

Cobalamin and 7 cobalamin analogues were identified and quantitated in human feces. The mean for the total amount present in 18 subjects whose daily intake was < or = 25 microg cobalamin from vitamin supplements was 1309 ng cobalamin equivalents/g wet wt of feces. Cobalamin (1.4%) and cobinamide (1.8%) (an incomplete corrinoid) represented a small portion of the total amount. Six cobalamin analogues that contain a base other than the 5,6-dimethylbenzimidizidole in cobalamin were present. The bases and their mean amounts (in %) are 2-methyladenine (60.6%), p-cresol (16.3%), adenine (12.5%), 2-(methylthio)adenine (15.5%), 5-hydroxybenzimidazole (1.8%), and phenol (0.1%). One subject ingested 1 mg cobalamin/d and another ingested 2 mg cobalamin/d, and they appeared to convert most of the cobalamin to cobinamide and the 4 analogues that contain the bases-2-methyladenine, p-cresol, adenine, and 2-(methylthio)adenine.

CONCLUSIONS

Cobalamin analogues are present in human feces and account for > 98% of the total of cobalamin plus cobalamin analogues. A major portion of large amounts of ingested cobalamin appears to be converted to cobalamin analogues.

Cobalamin as an analytical tool for analysis of oxirane metabolites of 1,3-butadiene: Development and validation of the method

The reduced form of vitamin B12 [cob(I)alamin] is known to be a supernucleophile, with the ability to react 10(5) times faster than standard nucleophiles. Procedures have been developed where cob(I)alamin is used as an analytical tool for the trapping of electrophilically reactive compounds. In the present work, a sensitive and accurate method for determination of reactive metabolites produced in vitro has been developed and validated. Diepoxybutane (DEB), a metabolite of 1,3-butadiene, was used as a model compound. The intermediate precursor 1,2-epoxybutene (EB) was incubated in a mouse liver S9 metabolic system and the formation of DEB was studied. Samples were taken at different times from the incubation mixture and added to the cob(I)alamin. The alkyl-cobalamins (alkyl-Cbl) formed were directly analysed by a miniaturized LC-MS/MS method and column switching. The assay was linear over the concentration range of 1.5-500 microM with acceptable precision and accuracy.

Reaction-kinetic parameters of glycidamide as determinants of mutagenic potency

Values for reaction-kinetic parameters of electrophiles can be used to predict mutagenic potency. One approach employs the Swain-Scott relationship for comparative kinetic studies of electrophilic agents reacting with nucleophiles. In this way glycidamide (GA), the putatively mutagenic/carcinogenic metabolite of acrylamide, was assessed by determining the rates of reaction with different nucleophiles. The rate constants (kNu) were determined using the "supernucleophile" cob(I)alamin [Cbl(I)] as an analytical tool. The Swain-Scott parameters for GA were compared with those of ethylene oxide (EO). The substrate constants, s values, for GA and for EO were found to be 1.0 and 0.93, respectively. The reaction rates at low values of nucleophilic strength (n=1-3), corresponding to oxygens in DNA, were determined to be 2-3.5 times higher for GA compared to EO. GA was also more reactive than EO towards other nucleophiles (n=0-6.4). The mutagenic potency of GA was determined in Chinese hamster ovary cells (hprt mutations in CHO-AA8 cells per dose unit with gamma-radiation as reference standard). The potency of GA was estimated to be about three mutations per 10(5) cells and mMh corresponding to about 40 rad-equ./mMh. A preliminary comparison of the mutagenic potency (per mMh and as rad-equivalents) of GA and EO shows an approximately seven times higher potency for GA. A higher mutagenic potency of GA compared to EO is compatible with expectation from reaction-kinetic data of the two compounds. The data confirmed that GA is not a strong mutagen, which is in line with what is expected for simple oxiranes. The present study shows the value of cob(I)alamin for the determination of reaction-kinetic parameters and their use for prediction of mutagenic potency.

A New Role for Glutathione: Protection of Vitamin B12from Depletion by Xenobiotics

NADPH in microsomes reduces the hydroxocob(III)alamin form of vitamin B12 to cob(II)alamin and the supernucleophilic cob(I)alamin, which are both highly reactive toward xenobiotic epoxides formed by mammalian metabolism of dienes such as the industrially important chemicals chloroprene and 1,3-butadiene. With styrene, the metabolically formed styrene oxide is reactive toward cob(I)alamin but not cob(II)alamin. Such reactions in humans could lead to vitamin B12 deficiency, which is implicated in pernicious anemia, cancer, and degenerative diseases. However, glutathione inhibits the reduction of hydroxocob(III)alamin by formation of the 1:1 complex glutathionylcobalamin. This blocks reactions of the cobalamins with metabolically formed epoxides. The interaction between glutathione and vitamin B12 could protect against diseases related to vitamin B12 depletion.

Characterization of alkyl-cobalamins formed on trapping of epoxide metabolites of 1,3-butadiene

Analytical methods facilitating studies of electrophilically reactive and genotoxic compounds in vitro and in vivo are needed. The strong nucleophile, cob(I)alamin, formed by reduction of Vitamin B12 [cob(III)alamin], may be used for trapping and analysis of 1,2-epoxides and other electrophiles. In the present study, cob(I)alamin is evaluated as an analytical tool for 1,2-epoxide metabolites (oxiranes) of 1,3-butadiene. Products of reaction of cob(I)alamin with 1,2-epoxy-3-butene (EB), 1,2:3,4-diepoxybutane (DEB), and 1,2-epoxy-3,4-butanediol (EBdiol) have been analyzed by reversed phase high performance liquid chromatography (HPLC) coupled on-line to electrospray ionization mass spectrometry (ESI-MS) and ultraviolet diode array detection (UV-DAD). It was shown that a specific alkyl-CbI complex is formed for each metabolite and that it was possible to discriminate between the products by HPLC-UV and by LC-MS. Quantification of DEB with the method by use of another 1,2-epoxide as an internal standard was successfully performed. The possibility of using cob(I)alamin for trapping and analysis of the three oxirane metabolites of 1,3-butadiene will facilitate quantitative comparisons of species in vitro with regard to metabolism of 1,3-butadiene.

Evidence for phosphate adducts in DNA from mice treated with 4-(N-Methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK)

The weakly alkylating capacity of phosphotriesters (PTE) has been used for the determination of adducts to phosphate groups in DNA by specific transfer to the strongly nucleophilic compound cob(I)alamin [Cbl(I)]. When enzymatically degraded liver DNA from mice treated with 1-(N-methyl-N-nitrosamino)-4-(3-[3H]pyridyl)-4-oxobutane ([3H]NNK) was added to Cbl(I), a 4-(3-[3H]pyridyl)-4-hydroxy-1-butyl-cobalamin ([3H]PHB-Cbl) complex was formed and determined by HPLC and liquid scintillation counting. The PHB-Cbl formed was compared with a synthetic standard verified by LC/MS and 1H NMR and corresponds to phosphate adducts formed from the pyridyloxobutylating species from NNK and from the pyridylhydroxybutylating species from NNAL, NNK being to a large extent converted to NNAL in vivo. It was concluded that about 22% of the total level of pyridyl (oxo or hydroxy) butyl adducts to DNA was bound to phosphate groups.