Molecular modeling of carcinogenic potential in polycyclic hydrocarbons

Reactions of deoxyribonucleotide bases with sulfooxymethyl or halomethyl polycyclic aromatic hydrocarbons induce unwinding of DNA supercoils

Torsional stress in double-stranded DNA enables and regulates facets of chromosomal metabolism, replication, and transcription and requires regulatory enzymatic systems including topoisomerases and histone methyltransferases. As such, this machinery may be subject to deleterious effects from reactive mutagens, including ones from carcinogenic polycyclic aromatic hydrocarbon (PAH) adduct formation with DNA. Supercoiled plasmid DNA was investigated for its torsional responses to adducts formed in vitro from PAH benzylic carbocation reactive intermediates created spontaneously by release of leaving groups. PAH sulfate esters were found to (1) unwind DNA in a concentration dependent manner, and (2) provide maximum unwinding in a pattern consistent with known carcinogenicities of the parent PAHs, that is, 6-methylbenzo[a]pyrene > 7,12-methylbenz[a]anthracene > 3-methylcholanthrene > 9-methylanthracene > 7-methylbenz[a]anthracene > 1-methylpyrene. Supercoil unwinding was demonstrated to be dependent on the presence of sulfate or chloride leaving groups such that reactive carbocations were generated in situ by hydrolysis. In silico modeling of intercalative complex topology showed PAH benzylic carbocation reactive functional groups in alignment with target nucleophiles on guanine bases in a 5'-dCdG-3' pocket in agreement with known formation of nucleotide adducts. Inhibitory or modulatory effects on PAH-induced supercoil unwinding were seen with ascorbic acid and an experimental antineoplastic agent Antineoplaston A10 in agreement with their known anticarcinogenic properties. In summary, the reactive PAH intermediates studied here undoubtedly participate in well-known mutational mechanisms such as frameshifts and apurinic site generation. However, they are also capable of random disruption of chromosomal supercoiling in a manner consistent with the known carcinogenicities of the parent compounds, and this mechanism may represent an additional detrimental motif worthy of further study for a more complete understanding of chemical carcinogenicity.

Bioremediation of a pentacyclic PAH, Dibenz(a,h)Anthracene- A long road to trip with bacteria, fungi, autotrophic eukaryotes and surprises

Dibenz(a,h)Anthracene (DBahA), classified as a probable human carcinogen (B2) is the first Poly Aromatic Hydrocarbons (PAH) to be chemically purified and used for cancer-based studies. Till date, only 30 papers focus on the bioremediation aspects of DBahA out of more than 200 research publications for each of the other 15 priority PAHs. Thus, the review raises an alarm and calls for efficient bioremediation strategies for considerable elimination of this compound from the environment. This article reviews and segregates the available papers on DBahA bioremoval from the beginning till date into bacteria, fungi and plant-mediated remediation and offers suggestions for the most competent and cost-effective modes to bioremove DBahA from the environment. One of the proficient ways to get rid of this PAH could with the use of biosurfactant-enriched bacterial consortium in DBahA polluted environment, which is given considerable importance here. Among the bacterial and fungal microbiomes, unquestionably the former are the beneficiaries which utilize the breakdown products of this PAH metabolized by the latter. Nevertheless, the use of plant communities for efficient DBahA utilization through fibrous root system is also discussed at length. The current status of DBahA as reflected by the publications at https://www.ncbi.nlm.nih.gov and recommendations among the explored groups [bacterial/fungal/plant communities] for better DBahA elimination are pointed out. Finally, the review emphasizes the pros and cons of all the methodologies used for selective/combinatorial removal of DBahA and present the domain to the researchers to carry forward by incorporating their individual ideas.

Structure, function and carcinogenicity of metabolites of methylated and non-methylated polycyclic aromatic hydrocarbons: a comprehensive review

The Unified Theory of PAH Carcinogenicity accommodates the activities of methylated and non-methylated polycyclic aromatic hydrocarbons (PAHs) and states that substitution of methyl groups on meso-methyl substituted PAHs with hydroxy, acetoxy, chloride, bromide or sulfuric acid ester groups imparts potent cancer producing properties. It incorporates specific predictions from past researchers on the mechanism of carcinogenesis by methyl-substituted hydrocarbons, including (1) requirement for metabolism to an ArCH2X type structure where X is a good leaving group and (2) biological substitution of a meso-methyl group at the most reactive center in non-methylated hydrocarbons. The Theory incorporates strong inferences of Fieser: (1) The mechanism of carcinogenesis involves a specific metabolic substitution of a hydrocarbon at its most reactive center and (2) Metabolic elimination of a carcinogen is a detoxifying process competitive with that of carcinogenesis and occurring by a different mechanism. According to this outlook, chemical or biochemical substitution of a methyl group at the reactive meso-position of non-methylated hydrocarbons is the first step in the mechanism of carcinogenesis for most, if not all, PAHs and the most potent metabolites of PAHs are to be found among the meso methyl-substituted hydrocarbons. Some PAHs and their known or potential metabolites and closely related compounds have been tested in rats for production of sarcomas at the site of subcutaneous injection and the results strongly support the specific predictions of the Unified Theory.

Formation of radical cations in a model for the metabolism of aromatic hydrocarbons

To test the hypothesis that electrophilic radical cations are the major ultimate electrophilic and carcinogenic forms of benz[a]anthracene (BA), dibenz[a,h]anthracene (DBA), and benzo[a]pyrene (BP), we have focused on a chemical model of metabolism which parallels and duplicates known or potential metabolites of some polycyclic hydrocarbons formed in cells. Studies of this model system show that radical cations are hardly formed, if at all, in the case of BA or DBA but are definitely formed in the cases of the carcinogen BP as well as the non-carcinogenic hydrocarbons, pyrene and perylene. We conclude that the carcinogenicities of BA, DBA, BP, pyrene, and perylene are independent of one-electron oxidation to radical cation intermediates.

Mass spectrometric analysis of 7-sulfoxymethyl-12-methylbenz[a]anthracene and related electrophilic polycyclic aromatic hydrocarbon metabolites

The Meso-region theory of polycyclic aromatic hydrocarbon (PAH) carcinogenesis predicts that the development of pronounced carcinogenicity depends on the introduction of a good leaving group on alkyl side-chains attached to the exceptionally reactive meso-anthracenic or L-region positions of PAHs. Thus, the first step in carcinogenesis by methylated PAHs such as 7,12-dimethylbenz[a]anthracene (DMBA) would be the hydroxylation of the L-region methyl groups, particularly the 7-methyl group. The second would be the formation of a metabolite, e.g. a sulfate ester, which is expected to be a good leaving group capable of generating a highly reactive benzylic carbocation. 7-Hydroxymethyl-12-methylbenz[a]anthracene (7-HMBA) is a metabolite of DMBA, and sulfation of 7-HMBA to a 7-sulfoxymethyl metabolite (7-SMBA) is a known Phase II metabolic process designed to facilitate excretion, but actually enabling more destructive side-reactions. These side-reactions occur with generation of an electrophilic 7-methylene carbonium ion, and/or by in vivo halide exchange to provide neutral side-products more capable of entering cells, especially those of DMBA target tissues. Electrospray ionization mass spectrometry (MS) enabled us to visualize 7-SMBA as an intact m/z 351 conjugate anion by negative mode, and as a released m/z 255 carbonium ion by positive mode. Upon prolonged refrigeration, 7-SMBA accumulated an m/z 383 photooxide, which appeared capable of re-evolving the starting material as visualized by tandem quadrupole MS, or MS/MS. The 7-SMBA carbonium ion provided interpretable fragments when studied by fragment ion MS/MS, including those representing the loss of up to several protons. Subtle differences in this property were encountered upon perturbing 7-SMBA, either by warming it at 37 degrees C for 2 h or by substituting the initial sulfoxy group with an iodo group. Side-reactions accounting for such proton losses are proposed, and are of interest whether they occur in the mass spectrometer, in solution or both; these proposals include acidity at the 12-methyl position and cyclization between the 12-methyl group and the adjacent C-1 position. It is also suggested that such side-reactions may comprise one route to relieving steric strain arising between the 12-methyl group and the angular benzo ring of 7-SMBA.

Comparative carcinogenicity of picene and dibenz[a,h]anthracene in the rat

Early carcinogenicity tests found no evidence of activity for picene but found considerable initiating and carcinogenic activity for dibenz[a,h]anthracene (DBA). More recent investigation suggested that both pentacyclics were complete carcinogens when administered as single sc injections in NMRI mice, despite findings that picene acted as neither an initiating nor promoting agent. To investigate this contradiction, the complete carcinogenicities of both isomers were compared by sc injection in female Sprague-Dawley rats. The results demonstrate that 1 micromol of DBA, administered three times weekly for 20 doses, induces sarcomas in all test animals by 33 weeks (100%). Similar treatment with picene did not induce sarcoma in any test animals by 37 weeks (0%). The present results agree with the earlier studies. It follows from these results that the predictions of the unified theory for the appearance of carcinogenic properties following administration of picene and dibenz[a,h]anthracene to test animals have been confirmed.

Carcinogenicity of 1-hydroxy-3-methylcholanthrene and its electrophilic sulfate ester 1-sulfooxy-3-methylcholanthrene in Sprague-Dawley rats

Previous experiments have demonstrated that the carcinogen 1-hydroxy-3-methylcholanthrene is a metabolite of 3-methylcholanthrene. 1-Sulfooxy-3-methylcholanthrene, prepared by chemical synthesis from 1-hydroxy-3-methylcholanthrene, was shown to be a direct acting electrophilic mutagen and DNA damaging agent. These results imply that 1-hydroxy-3-methylcholanthrene could be metabolically activated to an ultimate electrophilic and carcinogenic form of 1-hydroxy-3-methylcholanthrene and 3-methylcholanthrene in a reaction catalyzed by 3'-phosphoadenosine-5'-phosphosulfate-dependent sulfotransferase activity. 1-Hydroxy-3-methylcholanthrene and its aralkylating reactive ester, 1-sulfooxy-3-methylcholanthrene, were individually administered to groups of 12 female Sprague-Dawley rats at a 0.2 mumol dose, three times weekly, for 20 doses. 1-Sulfooxy-3-methylcholanthrene induced sarcomas at the site of injection in 8 of 12 rats (66%) by 52 weeks, whereas 1-hydroxy-3-methylcholanthrene induced sarcomas at the site of injection in 5 of 12 rats (42%) by 52 weeks. These results, taken together with the results of previous experiments, strongly support the hypothesis that the activated electrophilic mutagen 1-sulfooxy-3-methylcholanthrene plays a major role as an ultimate electrophilic and carcinogenic form of 1-hydroxy-3-methylcholanthrene, a major metabolite of 3-methylcholanthrene.

6-sulfooxymethylbenzo[a]pyrene is an ultimate electrophilic and carcinogenic form of the intermediary metabolite 6-hydroxymethylbenzo[a]pyrene

Previous experiments have demonstrated that the intermediary metabolite 6-hydroxymethylbenzo[a] pyrene (HMBP) can be activated to the electrophilic mutagen, 6-sulfooxymethylbenzo[a]pyrene (SMBP), by rat and mouse liver PAPS-dependent sulfotransferase activity or by chemical synthesis. This aralkylating metabolite and 6-hydroxymethylbenzo[a]pyrene were individually administered to groups of 12 female Sprague-Dawley rats, at a 0.2 micromol dose three times weekly for 20 doses. SMBP induced sarcomas at the site of injection in 12 of 12 rats by 33 weeks, whereas HMBP induced sarcomas at the site of injection in 12 of 12 rats by 31 weeks. These results, taken together with the results of previous studies, strongly support the hypothesis that the electrophilic mutagen SMBP accounts for most, if not all, of the complete carcinogenicity of the intermediary metabolite HMBP and probably at least some of the complete carcinogenicity of 6-methylbenzo[a]pyrene (MBP), 6-formylbenzo[a]pyrene (formylBP), and even benzo[a]pyrene (BP), all of which are metabolized to HMBP.

7-Sulfooxymethyl-12-methylbenz[a]anthracene is an exceptionally reactive electrophilic mutagen and ultimate carcinogen

The hypothesis was tested that an ultimate carcinogen of 7-hydroxymethyl-12-methylbenz[a]anthracene (HMBA), a major metabolite of 7,12-dimethylbenz[a]anthracene (DMBA), is a benzylic carbonium ion generated from an exceptionally reactive aralkylating metabolite, such as an electrophilic sulfate ester. In conformity with this hypothesis, sarcomas were rapidly induced in rats following repeated subcutaneous injection of HMBA (67%) or its electrophilic sulfate ester, sodium 7-sulfooxymethyl-12-methylbenz[a]anthracene (SMBA) (100%). It would appear from the results summarized here that the search for a carcinogenic metabolite of DMBA has been successful. In addition, an aralkylating electrophilic mutagen and carcinogen has been prepared from HMBA, which is itself either an ultimate carcinogen or a direct precursor of an ultimate carcinogen, i.e., a benzylic carbonium ion.