Photochemical coupling of benzo(a)pyrene with 1-methylcytosine; photoenhancement of carcinogenicity

Syncarcinogenesis of natural UV radiation and polycyclic aromatic hydrocarbons in the development of squamous cell carcinomas of the skin?

Squamous cell carcinomas (SCC) of the skin can be induced by occupational exposures to polycyclic aromatic hydrocarbons (PAHs), as in tar and soot, or to UV radiation and can be recognized and compensated as occupational diseases. A possible syncarcinogenic effect of these exposures in the development of SCC in humans is under discussion. For the scientific validation of this question, a systematic literature search was conducted using the databases PubMed, Web of Science, and Scopus. Studies on individuals with SCC of the skin and their precursors as well as occupational, non-occupational, or therapeutic exposure to UV radiation and PAHs were selected. In addition, animal studies with exposure to UV radiation and PAHs were evaluated. After screening the abstracts of 510 identified studies, the full texts of 131 studies were reviewed. None of the epidemiological studies provided robust evidence for a syncarcinogenesis of PAHs and UV radiation in the development of SCC of the skin in humans. Nevertheless, as there are indications for a (super-)additive effect of UV radiation and PAH exposure from animal studies and mechanistic investigations, syncarcinogenesis seems possible. However, quantitative dose-response relationships are lacking which would allow comparison of the onset of an adverse effect between the different exposure levels.

Luminescent charge-transfer complexes: tuning emission in binary fluorophore mixtures

Charge-transfer (CT) complexes composed of a π-electron-poor naphthalene diimide (NDI) derivative combined with a series of π-electron-rich donors were investigated. Solutions of the CT complexes are nonemissive; however, solid-state complexes and aqueous suspensions display emission that is dependent on the energy of the HOMO of the electron donor. Crystallographic analysis of a pyrene-NDI complex reveals columnar packing and a high degree of frontier molecular orbital (FMO) overlap that likely contributes to the observed optical properties. The fluorescent CT particles are utilized as imaging agents; additional luminescent CT complexes may be realized by considering FMO energies and topologies.

Activated toxicity of diesel particulate extract by ultraviolet a radiation in mammalian cells: role of singlet oxygen

BACKGROUND

Diesel exhaust [diesel exhaust particles (DEPs) and their extracts (DPE)] and ultraviolet A radiation (UVA) are two ubiquitous environmental factors that have been identified as essential risk factors for various benign or malignant human diseases, either alone or in combination with other agents.

OBJECTIVES

We aimed to investigate the synergistic effects of DPE and UVA at low-dose exposures in human-hamster hybrid (AL) cells and their underlying mechanisms.

METHODS

We exposed exponentially growing AL cells to DPE and/or UVA radiation with or without reactive oxygen species (ROS) quenchers and then assayed the cells for survival, mutation induction, apoptosis, and micronucleus generation. In addition, using a singlet oxygen (1O2) trapping probe, 2,2,6,6-tetramethyl-4-piperidone, coupled with electron paramagnetic resonance spectroscopy, we determined the production of 1O2.

RESULTS

Treatment of AL cells with DPE+UVA induced significant cytotoxic and genotoxic damage. In contrast, we found no significant damage in cells treated with either UVA or DPE alone at the same doses. Mutation spectra of CD59- mutants showed that treatment with DPE+UVA easily induces multilocus deletions. Sodium azide significantly inhibited both cellular and DNA damage induced by DPE+UVA treatment, whereas other ROS inhibitors had little protecting effect. Furthermore, we found a significant increase of 1O2 in the cells that received DPE+UVA treatment.

CONCLUSION

These findings suggest that UVA activated the genotoxicity and cytotoxicity of DPE in mammalian cells and that 1O2 played an important role in these processes.

DNA damage induced by coexposure to PAHs and light

Polycyclic aromatic hydrocarbons (PAHs) are widely distributed in the environment as pollutants in air, water and soil, and some are carcinogenic, being associated with various types of cancer. A majority of the research concerning the biological effects of PAHs has focused on the metabolic activation and DNA adducts leading to mutation and transformation. Although the role of the PAHs as photosensitizers has received much less attention, investigators have shown that PAHs excited by sunlight induced significant cytotoxicity and several kinds of DNA damage. Some PAHs were recently proved to be photomutagenic. In this review, we discuss the influence of PAHs in combination with sunlight focusing on the phototoxicity and cellular DNA damage produced.

Coexposure to benzo[a]pyrene and UVA induces DNA damage: first proof of double-strand breaks in a cell-free system

DNA damage induced by solar ultraviolet (UV) radiation plays an important role in the induction of skin cancer. Although UVA constitutes the majority of solar UV radiation, it is less damaging to DNA than UVB. The DNA damage produced by UVA radiation, however, can be augmented in the presence of a photosensitizer. We previously used benzo[a]pyrene (BaP), an environmental carcinogenic polycyclic aromatic hydrocarbon, as an exogenous photosensitizer, and demonstrated that combined exposure to BaP and UVA resulted in DNA double-strand breaks (DSBs) in cultured Chinese hamster ovary (CHO-K1) cells. In this study, we investigated whether coexposure to BaP and UVA induces DSBs in a cell-free system and whether reactive oxygen species (ROS) were involved in the generation of the DSBs. DSBs were induced by the coexposure both in the cell-free system (in vitro) and in CHO-K1 cells (in vivo), but not by treatment with BaP or UVA alone. DSB induction in vitro required higher doses of UVA and BaP than were required in vivo, suggesting that the mechanism of DSB induction differed. A similar difference in efficiency also was observed in the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) by coexposure to BaP and UVA in vitro and in vivo. A singlet oxygen ((1)O2) scavenger (NaN3) effectively inhibited the production of DSBs and 8-oxodG, suggesting that (1)O2 is a principal ROS generated by BaP and UVA both in vitro and in vivo. Furthermore, repair-deficient xrs-5 cells were more sensitive to coexposure with BaP and UVA than were CHO-K1 cells, but the two cell lines were equally sensitive to the combined treatment in the presence of NaN3. This result suggested that the cell death produced by coexposure to BaP and UVA was at least partly due to the DSBs generated by (1)O2. Our findings indicate that coexposure to BaP and UVA effectively induced DNA damage, especially DSBs, which results in phototoxicity and possibly photocarcinogenesis.

Light-dependent mutagenesis by benzo[a]pyrene is mediated via oxidative DNA damage

Benzo[a]pyrene (B[a]P) is an environmental carcinogenic polycyclic aromatic hydrocarbon (PAH). Mammalian enzymes such as cytochrome P-450s and epoxide hydrase convert B[a]P to reactive metabolites that can covalently bind to DNA. However, some carcinogenic compounds that normally require metabolic activation can also be directly photoactivated to mutagens. To examine whether B[a]P is directly mutagenic in the presence of light, we exposed Salmonella typhimurium strains with different DNA repair capacities to B[a]P and white fluorescent light at wavelengths of 370-750 nm. B[a]P plus light significantly enhanced the number of His+ revertants. Mutagenesis was completely light-dependent and required no exogenous metabolic activation. The order of mutability of strains with different DNA repair capacities was strain YG3001 (uvrB, mutMST) >> strain TA1535 (uvrB) > strain YG3002 (mutMST) > strain TA1975. The uvrB gene product is involved in the excision repair of bulky DNA adducts, and the mutMST gene encodes 8-oxoguanine (8-oxoG) DNA glycosylase, which removes 8-oxoG from DNA. Introduction of a plasmid carrying the mOgg1 gene that is the mouse counterpart of mutMST substantially reduced the light-mediated mutagenicity of B[a]P in strain YG3001. B[a]P plus light induced predominantly G:C --> T:A and G:C --> C:G transversions. We propose that B[a]P can directly induce bulky DNA adducts if light is present, and that the DNA adducts induce oxidative DNA damage, such as 8-oxoG, when exposed to light. These findings have implications for the photocarcinogenicity of PAHs.

Environmental carcinogenic polycyclic aromatic hydrocarbons: photochemistry and phototoxicity

Polycyclic aromatic hydrocarbons (PAHs) are a class of environmental contaminants that has long been of interest in the fields of organic chemistry, theoretical chemistry, physical chemistry, environmental science, toxicology, cancer research, and energy sciences. Concerning environmental science and cancer research, majority of the research has focused on the occurrence, environmental fate, degradation/remediation, chemical transformation, genotoxicity, metabolism and metabolic activation, DNA adduct formation, mutagenesis, and carcinogenesis. Although many books and reviews on these subjects have been published, PAH photochemistry and phototoxicity have received much less attention. Therefore, it is intended for this article to provide an up-to-date source of photochemical reaction, photo-transformation, and phototoxicity of PAHs and their oxygenated, nitrated, halogenated, and amino substituted derivatives on a molecular basis. A perspective for future work is also discussed.

Biotransformation of benzo[a]pyrene and other polycyclic aromatic hydrocarbons and heterocyclic analogs by several green algae and other algal species under gold and white light

This laboratory has shown that the metabolism of benzo[a]pyrene (BaP), a carcinogenic polycyclic aromatic hydrocarbon (PAH), by a freshwater green alga, Selenastrum capricornutum, under gold light proceeds through a dioxygenase pathway with subsequent conjugation and excretion. This study was undertaken to determine: (1) the effects of different light sources on the enzymatic or photochemical processes involved in the biotransformation of BaP over a dose range of 5-1200 mg/l; (2) the phototoxicity of carcinogenic PAHs and mutagenic quinones to a green alga; (3) the ability of other algal systems to metabolize BaP. Cultures were exposed to different doses of BaP for 2 days at 23 degrees C under gold, white or UV-A fluorescent light on a diurnal cycle of 16 h light, 8 h dark. Under gold light, metabolites of BaP produced by Selenastrum capricornutum were the dihydrodiols of which the 11,12-dihydrodiol was the major metabolite. Under white light, at low doses, the major metabolite was the 9,10-dihydrodiol. With increasing dose, the ratio of dihydrodiols to quinones decreased to less than two. With increasing light energy output, from gold to white to UV-A in the PAH absorbing region, BaP quinone production increased. Of other carcinogenic PAHs studied, only 7H-dibenz[c,g]carbazole was as phototoxic as BaP while 7,12-dimethylbenz[a]anthracene, dibenz[a,j]acridine and non-carcinogenic PAHs, anthracene and pyrene, were not phototoxic. The 3,6-quinone of BaP was found to be highly phototoxic while quinones that included menadione, danthron, phenanthrene-quinone and hydroquinone were not. The data suggest that the phototoxicity of BaP is due to photochemical production of quinones; the 3,6-quinone of BaP is phototoxic and is probably the result of the production of short lived cyclic reactive intermediates by the interaction of light with the quinone. Lastly, only the green algae, Selenastrum capricornutum, Scenedesmus acutus and Ankistrodesmus braunii almost completely metabolized BaP to dihydrodiols. The green alga Chlamydomonas reinhardtii, the yellow alga Ochromonas malhamensis, the blue green algae Anabaena flosaquae and euglenoid Euglena gracilis did not metabolize BaP to any extent. The data indicate that algae are important in their ability to degrade PAHs but the degradation is dependent on the dose of light energy emitted and absorbed, the dose of PAHs to which the algae are exposed, the phototoxicity of PAHs and their metabolite(s) and the species and strain of algae involved. All of these factors will be important in assessing the degradation and detoxification pathways of recalcitrant PAHs by algae.

Central role of radical cations in metabolic activation of polycyclic aromatic hydrocarbons

1. Development of the chemistry of polycyclic aromatic hydrocarbon (PAH) radical cations has provided evidence that these intermediates play a major role in the metabolism of PAHs by P450 and in their binding to DNA. 2. Fluoro substitution of benzo[a]pyrene (BP) represents a suitable probe for studying mechanisms of oxygen transfer in the P450-catalysed formation of quinones and phenols from BP. Formation of BP-1,6-, -3,6- and -6,12-dione from the metabolism of 6-fluoroBP (6-FBP) is mediated by the intermediate 6-FBP+. Similarly, metabolism of 1-FBP and 3-FBP by rat liver microsomes produces BP-1,6-dione and BP-3,6-dione respectively. These results demonstrate that formation of quinones and phenols occurs via an initial electron transfer from BP to P450 and subsequent transfer of oxygen from the iron-oxo complex of P450 to BP. 3. Radical cations also play a major role in the formation of DNA adducts by the potent carcinogens 7,12-dimethylbenz[a]anthracene (DMBA), BP and dibenzo[a,l]pyrene (DB[a,l]P). In the binding of BP both in vitro and in vivo, 80% of the adducts are formed by one-electron oxidation, namely, 8-(BP-6-yl)guanine (BP-6-C8Gua), BP-6-N7Gua and BP-6-N7adenine (Ade), and are lost from the DNA by depurination. For DB[a,l]P, depurinating adducts formed from the radical cation, DB[a,l]P-10-C8Gua, DB[a,l]P-10-N7Gua, DB[a,l]P-10-N7Ade, and DB[a,l]P-10-N3Ade comprise 50% of the total DNA adducts. For DMBA, 99% of the adducts are depurinating adducts formed from the radical cation, 7-CH3BA-12-CH2-N7Gua and 7-CH3BA-12-CH2-N7Ade. 4. In summary, radical cations of PAHs play a major role in both the metabolism and metabolic activation leading to formation of DNA adducts that are critical in the mechanism of tumour initiation.

The approach to understanding aromatic hydrocarbon carcinogenesis. The central role of radical cations in metabolic activation

Polycyclic aromatic hydrocarbons (PAH) are carcinogens requiring metabolic activation to react with cellular macromolecules, the initial event in carcinogenesis. Cytochrome P450 mediates binding of PAH to DNA by two pathways of activation. One-electron oxidation to form radical cations is the major pathway of activation for the most potent carcinogenic PAH, whereas monooxygenation to form bay-region diol epoxides is generally a minor pathway. For benzo[a]pyrene and 7,12-dimethylbenz[a]-anthracene, 80% and 99%, respectively, of the DNA adducts formed by rat liver microsomes or in mouse skin arise via the radical cation. Therefore, studies of PAH activation should begin by considering one-electron oxidation as the primary mechanism.

Free radicals as carcinogens and their quenchers as anticarcinogens

An oxygen dependent signal was detected, late in the 1950s by electron spin resonance (ESR) in a saline solution of hematoporphyrin (Hp) excited by light. This signal expressed a free radical consisting of 'some kind of an association between Hp and oxygen', that Smaller et al. called 'oxyradical' (HpOO.). It soon opened a new level of understanding in carcinogenesis triggered by photodynamic substances, including Hp itself, polycyclic hydrocarbons (PCHs), as well as any carcinogen involving molecular species activated by radiation and/or metabolic reaction. Early in the 1960s, this prompted the discovery of benzo(a)pyrene (BP) photocarcinogenic enhancement (BP-PCE) in mice, probably due to an increase in free oxygen radical generation following correct light exposure. This assumption was confirmed in 1980 by the fact that mice orally loaded with antioxidants and radical quenchers, such as beta-carotene (BC) and cantaxanthin (CX), were protected against BP-PCE at 100% and against total BP carcinogenicity at more than 60%. These achievements were presented as the bases of the current explosion of interest in biology and medicine in building up the new field of chemoprevention against cancer and other chronic diseases by supplementation with antioxidant vitamins, retinoids and especially carotenoids and their synergistic association. The relevant findings of this research obtained in the last decade in in vitro and in vivo experiments as well as human interventions are reported and discussed with personal contributions.

The light-dependent cytotoxicity of benzo[a]pyrene: effect on human erythrocytes, Escherichia coli cells, and Haemophilus influenzae transforming DNA

In vitro, the photodynamic compound benzo[a]pyrene (BAP) generates singlet oxygen efficiently when irradiated in organic solvents. It also photogenerates superoxide anion radical in water and can act as a photoreducing agent in the absence of oxygen. In vivo, the hemolysis of human erythrocytes, the inactivation of Escherichia coli cells representing a series of strains differing in excision repair and catalase proficiency, and the inactivation of Haemophilus influenzae transforming DNA activity were used to characterize the phototoxicity of BAP in the presence of near-UV light (290-400 nm). The results are consistent with BAP behaving as a photosensitizer that generates both superoxide and singlet oxygen, and that damages chiefly membranes. DNA does not seem to be a major target in the phototoxic reactions investigated.

Metabolism of mutagenic polycyclic aromatic hydrocarbons by photosynthetic algal species

Polycyclic aromatic hydrocarbons (PAH) known to produce carcinogenic and mutagenic effects have been shown to contaminate waters, sediments and soils. While it is accepted that metabolites of these compounds are responsible for most of their biological effects in mammals, their metabolism, and to a large extent their bioactivity, in aquatic plants have not been explored. Cultures of photosynthetic algal species were assayed for their ability to metabolize benzo[a]pyrene (BaP), a carcinogenic PAH under conditions which either permitted (white light) or disallowed (gold light) photooxidation of the compound. Growth of Selenastrum capricornutum, a fresh-water green alga, was completely inhibited when incubated in white light with 160 micrograms BaP/l medium. By contrast concentrations at the upper limit of BaP solubility in aqueous medium had no effect on algal growth when gold light was used. BaP quinones and phenol derivatives were found to inhibit growth of Selenastrum under white light incubation. BaP phototoxicity and metabolism were observed to be species-specific. All 3 tested species of the order Chlorococcales were growth-inhibited by BaP in white light whereas neither the green alga Chlamydomonas reinhardtii nor a blue-green, a yellow-green or an euglenoid alga responded in this fashion. Assays of radiolabeled BaP metabolism in Selenastrum showed that the majority of radioactivity associated with BaP was found in media as opposed to algal cell pellets, that the extent of metabolism was BaP concentration dependent, and that the proportion of various metabolites detected was a function of the light source. After gold light incubation, BaP diols predominated while after white light treatment at equal BaP concentrations, the 3,6-quinone was found in the highest concentration. Extracted material from algal cell pellets and from media was tested for mutagenicity in a forward mutation suspension assay in Salmonella typhimurium using resistance to 8-azaguanine for selection. Direct-acting mutagens were detected in extracted media from incubation of Selenastrum with 400 micrograms BaP/l for 1 day in gold light. Extracts of media from algae incubated in gold light from 1 to 4 days with 1200 micrograms BaP/l were found to have direct-acting mutagens as well as those requiring further metabolism. Media extracts from white light incubations of BaP were mutagenic upon addition of rat liver homogenates. Activity of these materials from white light treatment are largely attributable to unmetabolized BaP.

The phototoxicity of benzo[a]pyrene in the green alga Selenastrum capricornutum

The effects of selected polycyclic aromatic hydrocarbons (PAHs) on the growth of the green alga Selenastrum capricornutum in three light regimens were examined. In gold fluorescent light, benzo[a]pyrene (BaP) at 12 mg/liter (48 mumole/liter), benz[a]anthracene (BaA) at 40 mg/liter (175 mumole/liter), anthracene (A) at 40 mg/liter (224 mumole/liter), and 13 metabolites of BaP each at 40 micrograms/liter had no effect on algal growth. In cool-white fluorescent light, 30% inhibition of algal growth occurred with 0.1 mumole/liter BaP, 8.0 mumole/liter BaA, and 40 mumole/liter A. BaP at 0.16 mg/liter (0.64 mumole/liter) totally inhibited growth. BaP concentrations an order of magnitude lower inhibited algal growth in fluorescent blacklight. In cool-white light, 5 of 13 metabolites of BaP (each 40 micrograms/liter) inhibited algal growth: 3,6-quinone; 6-hydroxy; 9-hydroxy; 3-hydroxy; and 1,6-quinone. Based on these results, PAHs and metabolites of BaP are selectively phototoxic to S. capricornutum due to the incident light intensity below 550 nm.

Photocarcinogenicity of 8-methoxypsoralen and aflatoxin B1 with longwave ultraviolet light

The carcinogenicity of 8-methoxypsoralen (8-MOP) and aflatoxin B1 (AFB1) with longwave ultraviolet light (UVA) to hairless mouse skin was investigated. Skin tumors were induced efficiently by 8-MOP + UVA, and the time to 50% tumor incidence was about 24 weeks. Histopathologically, some tumors were squamous cell carcinomas. AFB1 did not show any phototoxic and photocarcinogenic effects on mouse skin in this study. Although the structure and the photoreactivity of AFB1 to DNA were similar to those of 8-MOP, the photocarcinogenic response of these compounds to mouse skin was quite different.

Presence of benzo[a]pyrene and other polycyclic aromatic hydrocarbons in suntan oils

Column and thin-layer chromatography followed by spectrophotofluorometry were used to determine the carcinogen benzo[a]pyrene and other polycyclic aromatic hydrocarbons (PAH) in commercial samples of four suntan iols based on mineral and/or vegetable oils liable to contain traces of PAH. The analyses showed that all the samples contained benzo[a]pyrene together with three other mutagenic, co-carcinogenic or non-carcinogenic PAH (perylene, fluoranthene and benzo]k]fluoranthene). One sample also contained the carcinogen anthanthrene. The total PAH content of the samples varied from 88 . 5 to 188 . 7 ng/g while benzo[a]pyrene levels were in the 1 . 5-4 . 7 ng/g range. The results suggest that users of suntan oils may be exposed to low levels of potentially hazardous PAH; however, in comparison with many other cosmetics that are presumably based on similar oils, suntan oils are only used intermittently and for relatively short periods.

Reactions induced in vitro between model DNA and benzo[a]pyrene by near-ultraviolet radiation

Near-ultraviolet (300--480 nm wavelength) irradiation of the single-strand polydeoxynucleotide poly[d(A,C,G,T)] and carbon-14 labeled benzo[alpha]-pyrene (B[alpha] P) in aqueous dimethylsulfoxide (DMSO) solution led to appreciable binding of labeled hydrocarbon to the polynucleotide. Nuclease digests of polydeoxynucleotide-B[alpha]P complexes were examined by chromatography on Sephadex LH-20; at high fluences of near-ultraviolet light deoxyguanosine (dG) residues of the polymer were largely destroyed when the hydrocarbon was present. Approximately 85% of the B[alpha] P of the digests were recovered as hydrophilic derivatives not adsorbed by Sephadex LH-20. Elution of the columns with an aqueous-methanol gradient indicated that substances similar to the covalent deoxynucleoside-B[alpha] P adducts formed between microsomally-oxidized B[alpha] P and DNA were likewise present in the digests. When the deoxyadenosine (dA), deoxycytidine (dC) or dG moieties of the polymer were tritium-labeled, substances doubly-labeled with tritium and carbon-14 were found; ratios of the two radioactivities indicated that equimolar amounts of deoxynucleoside and hydrocarbon were present.

Mutation of Chinese hamster cells by near-UV activation of promutagens

A tissue-culture assay for mutagenesis and cytotoxicity incorporating near ultraviolet (NUV) light activation of polyaromatic hydrocarbons (PAH) has been developed. Cultures of Chinese hamster cells (line CHO) growing in suspension culture were inoculated with benzo[a]pyrene (B[a]P), 7,12-dimethylbenzanthracene (DMBA) of shale-oil retort-water and exposed to light from a high-pressure mercury lamp fitted with a Corning NUV bandpass filter. This light source both permitted activation of PAH and the shale-oil water and preculded detectable damage to DNA. Neither the PAH nor the NUV alone had any effect on cell survival or mutation frequencies but the chemicals plus NUV were extremely effective in producing mutations to 6-thioguanine resistance (hgprt gene).

Acute skin phototoxicity in hairless mice following exposure to crude shale oil or natural petroleum oil

The acute skin phototoxicity in hairless mice resulting from applications of crude shale oil or natural petroleum oil and exposure to near ultraviolet light has been investigated. In addition, the absorption and excitation-emission spectra of these oils have been described. While the application of crude shale oil alone was shown to elicit acute skin damage, the combination of the crude shale oil and near ultraviolet light induced skin damage much more severe than that produced by either agent alone. In contrast, the application of the natural petroleum oil and near ultraviolet light was less toxic than that of the crude shale oil in the absence of light. These results suggest that a potential health hazard exists from contact with shale oil compounds and exposure to near ultraviolet light from either the sun or artificial sources.

Photooxidation products of 7,12-dimethylbenz[a]anthracene

The principal products of the photooxidation of 7,12-dimethylbenz[a]-anthracene (DMBA) in aqueous solutions by photooxidation induced by laboratory lighting have been characterized by high performance liquid chromatograms (HPLC), ultraviolet and mass spectrograms and by comparisons with authentic samples. The products identified were the 7,12-epidioxy-7,12-dihydro-7-12-dimethyl-, 7,12-dione, 7-hydroxymethyl-12-methyl-, 12-hydroxymethyl-7-methyl-, 7-formyl-12-methyl-, 12-formyl-7-methyl-, and 12-hydroxy-12-methyl-7-one derivatives of benz[a]-anthracene. The HPLC profile of products is similar to that obtained from oxidation of DMBA by 'one-electron' reagents, singlet oxygen, or liver microsomal metabolism. The first points of attack are the 7- and 12- positions. The mechanism of photooxidation appears to be generation of singlet oxygen by photodynamic effect of DMBA. None of the products is photosensitizing, however.

Photoproducts from DNA pyrimidine bases and polycyclic aromatic hydrocarbons

The major photoproduct formed between benzo[a]pyrene and thymine is identified as 1-(benzo[a]pyren-6-yl)-thymine by means of spectroscopic analysis and isotopic syntheses. Irradiation of 1-methylcytosine hydrochloride and anthracene gives two isolable photoproducts of which one is assigned the structure 5-(anthracen-9-yl)-1-methylcytosine.

The binding of benz(a)anthracene derivatives to glyceraldehyde-3-phosphate dehydrogenase and mammary tissue following exposure to laboratory light

7,12-Dimethylbenz(a)anthracene (DMBA) and 7-methoxymethyl-12-methylbenz(a)anthracene (MeO-DMBA) are converted to a number of products during short exposures in aqueous suspension to laboratory illumination. The mixture of products binds to glyceraldehyde-3-phosphate dehydrogenase (GPDH) while inhibiting its activity but there is no apparent relationship between the binding and inhibition of enzyme activity. There is little, or no, binding or enzyme inhibition when the compounds are protected from light. 7-Bromomethyl-12-methylbenz(a)anthracene (Br-DMBA) binds to GPDH whether photoactivated or not but enzyme inhibition depends upon light exposure. The binding of light-exposed DMBA by surviving rat mammary tissue was five-times greater than with the unchanged hydrocarbon. Binding of MeO-DMBA products also occurred after light exposure but not in the dark.

Carcinogenesis

An examination has been made of the major chemicals which are known to be carcinogenic, either in animals or in humans. As a result, a generalized type of chemical property seems to be a prerequisite for the carcinogenic activity. In most cases, this is some kind of reactive electrophilic intermediate produced directly from the carcinogen or from one of its immediate metabolites. This electrophilic reagent is generally a positively charged ion of some kind. In the case of the hydrocarbons it seems to be an incipient carbonium ion; in the case of nitrogen compounds, it is a corresponding nitronium ion. There are a number of possible substrate materials (nucleophilic) in a cells, but the dominant one which has been suspected is the nucleic acid component, either the DNA or the RNA. The bases of these materials all have extra pi electrons susceptible to attach by the electrophiles of the carcinogen. Such a modification, particularly of the DNA, could lead to either major or minor changes in the genetic composition of the cell. Minor changes are usually rectifiable or are not visible, and this would include a large variety of point mutations.

The photocarcinogenicity of anthracence: photochemical binding to deoxyribonucleic acid in tissue culture

Anthracene becomes covalently bound to high-molecular-weight DNA in mammalian tissue culture as a result of irradiation at 365 nm after the incubation of cells with the hydrocarbon. At high radiation doses, the extent of binding exceeds one hydrocarbon molecule per 10(3) bases, and is lethal. At low radiation doses, much decreased binding is observed, but a majority of cells remain viable and can be recultured.

The inhibition of glyceradehyde-3-phosphate dehydrogenase by benz(a)anthracene and its derivatives after exposure to laboratory lighting

Colloidal dispersions of polycyclic aromatic hydrocarbons were found to inhibit glyceraldehyde-3-phosphate dehydrogenase (GPDH) and lactic acid dehydrogenase (LDH) after a short exposure of the incubation medium to laboratory lighting. The activation effect was observed with substituted 7-methyl derivatives of 12-methylbenz(a)anthracene also. One minute exposure of 7-methoxymethyl-12-methylbenz(a)anthracene (MeO-DMBA) to light was sufficient to produce a measurable amount of GPDH inhibition. The rate of enzyme inhibition was related to the pH of the dispersion. A small but significant effect of light on the inhibition of the enzymes by K-region epoxides was seen. Protein binding of the epoxides was not well correlated with the inhibition. The light effect was oxygen-dependent and was related to the hydrocarbon concentration. The active form decayed over a period of more than 25 days. The necessity for protecting reactions involving hydrocarbons and their derivatives from light is emphasized.