PubMed Health⌕ Search

Biomedical subjects

M M Coombs

Publications and source records attributed to M M Coombs.

At least 37 records · Page 2Linked to original sources
PubMed1986

Bay region distortions in cyclopenta[a]phenanthrenes.

Two newly synthesized cyclopenta[a]phenanthrenes, namely the 1-methyl (VIII) and 7,11-dimethyl (VII) derivatives of the parent ketone 15,16-dihydrocyclopenta[a]phenanthren-17-one (I), have been tested for their capacity to produce skin tumors in mice. The former (VIII) is essentially inactive, whereas the latter (VII) is very potent in both repeated application and two-stage tests. X-ray crystallographic structure analyses have been carried out on seven derivatives of (I), namely its 11-methyl (II), 11,12-dimethyl (III), 11-methoxy (V), 11-ethyl (VI) and 7,11-dimethyl (VII) analogues (carcinogens), the 1-methyl derivative (VIII), and 11,12,15,16-tetrahydro-11-methyl-17-oxocyclopenta[a]phenanthrene (IV) (both non-carcinogens). The detailed molecular structures resulting from these studies have shown the effects of steric interactions and substitutions on the bay-region geometry. The methyl group on C(11) causes distortions of the molecule in the bay region. Out-of-plane distortions in the bay regions of the 11-methyl derivatives (II, III, VII) are greater than for the 11-methoxy or the 11-ethyl derivatives (V, VI). Molecules (except for III and IV) are packed in the crystals with interactions that include C = O...H interactions; this packing is in layers that are nearly parallel to each other. A hydrogen atom of the 11-methyl group appears, from computer modeling, to interact sterically with the hydrogen atom of the bay-region expoxide group in the activated diol-epoxide; this steric interaction may force one conformer of the diol-epoxide to be the predominant form, thereby accounting for the importance of a bay-region methyl group. Further computer modeling has been used to analyze possible modes of interaction of the diol-epoxides of cyclopenta[a]phenanthrenes with DNA.

Animals↗
PubMed1985

A comparative examination of the in vitro metabolism of five cyclopenta[a]phenanthrenes of varying carcinogenic potential.

Metabolites of 15,16-dihydrocyclopenta[a]phenanthren-17-one and its 1- and 12-methyl homologues (all non-carcinogens) along with those from the 11-methyl and 11,12-dimethyl-17-ketones (carcinogens), produced in vitro by hepatic microsomes from methylcholanthrene induced rats, were separated by reverse phase h.p.l.c. Identifications of individual metabolites were based upon elution times, u.v. spectra, and in some cases by mass spectrometry, circular dichroism, and identity with synthetic derivatives. All five compounds were biologically oxidised at their terminal A and D rings to yield 1,2-dihydrodiols and 15- and 16- ols; with the exception of the 1-methyl compound, all also gave similar amounts of 3,4-dihydrodiols. The 1-methyl compound by contrast failed to produce this metabolite, furnishing instead the 4-phenol and five other, probably related phenolic derivatives. Previous work has established that for the 11-methyl-17-ketone, the 3,4-dihydrodiol is the proximate carcinogen. Thus, whereas lack of biological activity with the 1-methyl compound can be ascribed to its failure to produce a 3,4-dihydrodiol, in the case of the unsubstituted parent ketone and its 12-methyl derivative other determining factors must come into play.

Animals↗
PubMed1985

Comparison of the binding of some carcinogenic and non-carcinogenic cyclopenta[a]phenanthrenes to DNA in vitro and in vivo.

After microsomal activation in vitro, both the strong carcinogen 15,16-dihydro-11-methylcyclopenta[a]phenanthren-17-one and its inactive 12-methyl isomer bind covalently to added DNA, in the ratio approximately 6:1 (1595 and 254 mu mol/mol of DNA phosphorus, respectively). Over a thousand times less binding was observed when DNA was isolated from the skin of groups of mice that had received a topical dose of 1000 nmol of these two compounds, and of the unsubstituted ketone (non-carcinogen) and its 11,12-dimethyl derivative (carcinogen), 48 h previously; the binding ratios were 458, 155, 19 and 974 nmol/mol DNA phosphorus, respectively. Covalent binding of the 11-methyl compound to mouse skin DNA, measured 48 h after topical application, was linear with dose over the range 50 - 1000 nmol. It has previously been demonstrated for groups of mice initiated with 200 or 400 nmol of this carcinogen and promoted by repeated application of croton oil that skin tumour incidences were 50 and 70%, respectively. For a topical dose of 1000 nmol of the inactive 12-methyl isomer DNA binding in skin falls in this range, equivalent to DNA binding given by approximately 340 nmol of the carcinogen. Thus although the inactive unsubstituted parent compound essentially fails to bind to skin DNA after topical application, there does not seem to be a consistent relationship between extent of DNA binding in vivo and carcinogenicity among these cyclopenta[a]phenanthrenes. However, loss of the 12-methyl adducts from skin followed a logarithmic course over the first 10 days following application, with a half life of 3.5 days. In contrast, it was previously shown that removal of the 11-methyl adducts could not be measured above the normal rate of DNA turnover for this mouse tissue (half life, 6-7 days). It is suggested that active DNA repair of the 12-methyl lesions may contribute to the lack of activity of this isomer.

Animals↗
PubMed1985

Mechanism of mouse skin tumor promotion by chrysarobin.

The skin tumor-promoting ability of 1,8-dihydroxy-3-methyl-9-anthrone (chrysarobin) was compared with that of 12-O-tetradecanoylphorbol-13-acetate (TPA) and 1,8-dihydroxy-9-anthrone (anthralin) in SENCAR mice. Although dose-response comparisons indicated that chrysarobin was several orders of magnitude less potent than TPA for promoting papilloma formation, this anthrone was 1.5 to 2 times more potent than anthralin. Maximal papilloma responses were achieved by 15 weeks of promotion with TPA whereas at least 25 weeks of promotion were necessary to achieve maximal papilloma responses with chrysarobin or anthralin indicating marked differences in tumor latency between the two classes of compounds. Interestingly, at optimal promoting doses, chrysarobin gave a carcinoma response (22% with 0.3 carcinomas per mouse at 45 weeks) similar to that of TPA suggesting that this compound may be more efficient at promoting carcinomas than papillomas. In two-stage promotion experiments, chrysarobin was incapable of functioning independently as a Stage I or II promoter despite its complete promoting activity. Chrysarobin and TPA were compared at optimal promoting doses for their ability to induce: (a) skin edema, (b) epidermal hyperplasia, and (c) epidermal ornithine decarboxylase. In each case, distinct differences were noted between the two compounds. When taken together, the data support the hypothesis that anthracene-derived skin tumor promoters work at least in part by a mechanism different from the phorbol esters.

Animals↗
PubMed1984

Frequency of sister chromatid exchanges in human lymphocytes cultivated with a human hepatoma cell line as an indicator of the carcinogenic potency of two cyclopenta[a]phenanthrenes.

We demonstrate here that the carcinogen 15,16-dihydro-11-methylcyclopenta[a]phenanthren-17-one can cause sister chromatid exchange in human lymphocytes as well as it can cause mutation in bacterial cells and in V79 hamster cells. The non-methylated parent compound which has no tumorigenic action and yet significantly mutates both Salmonella typhimurium TA 100 and hamster V79 cells, has no effect on the frequency of sister chromatid exchange. These results support the idea that sister chromatid exchanges are a valuable additional indicator of tumorigenic potential.

Animals↗
PubMed1984

The effect of methyl substituents on the in vitro metabolism of cyclopenta[a]phenanthren-17-ones: implications for biological activity.

The in vitro metabolism of 15,16-dihydrocyclopenta[a]phenanthren-17-one and its 11- and 12-methyl derivatives has been compared. All three compounds form trans-3,4-dihydrodiols having quasi-diequatorial conformations and 3R,4R configurations. The trans-3,4-dihydrodiol of the mutagenic, but non-tumorigenic 15,16-dihydrocyclopenta[a]phenanthren-17-one appears to undergo stereospecific epoxidation to a syn-diol epoxide. By contrast the 3,4-dihydrodiol of the nontumorigenic 15,16-dihydro-12-methylcyclopenta[a]phenanthren-17-one appears to undergo stereospecific epoxidation to an anti diol-epoxide equivalent to that generated from 15,16-dihydro-11-methylcyclopenta[a]phenanthren-17-one which is a strong carcinogen. These results are discussed with reference to the biological activities of the parent compounds.

Animals↗
PubMed1984

The biological activity and activation of 15,16-dihydro-1,11-methanocyclopenta[a]phenanthren-17-one, a carcinogen with an obstructed bay region.

The proposal that an unobstructed bay region is a prerequisite for tumorigenic activity in cyclopenta[a]phenanthrene-17-ones is not supported by the observation of the tumorigenicity of 15,16-dihydro-1,11-methanocyclopenta[a]phenanthrene-17-one towards the skin of T.O. mice. The title compound is oxidised in vitro by a mixed function oxidase to produce, inter alia, a trans-3,4-dihydrodiol, postulated as the proximate tumorigen. Unequivocal identification of a second metabolite as a trans-1,2-dihydrodiol derivative demonstrates the potential for enzymatic oxidation within the obstructed bay region and supports the proposal that the ultimate tumorigen is a trans-3,4-dihydrodiol-anti-1,2-oxide. This is further substantiated by the chromatographic behaviour of the major hydrocarbon-nucleoside adduct derived from mouse skin treated with the parent compound in vivo. The structures of certain others of the metabolites produced in vitro are also considered.

Animals↗
PubMed1983

X-ray structural studies and molecular orbital calculations (CNDO/2) in a series of cyclopenta[a]phenanthrenes: attempts at correlation with carcinogenicity.

Comparative X-ray crystallographic structure analyses have been carried out on seven cyclopenta[a]phenanthrenes, namely 15,16-dihydocyclopenta[a]phenanthren-17-one and its 2-, 6- and 12-methyl homologues (non-carcinogens) and the 7-and 11-methyl and 1,11-methano derivatives (carcinogens). All-valence-electron molecular-orbital calculations by the CNDO/2 method, using the crystallographic parameters, have also been executed. Charge distribution and the energies of the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) have been calculated. With one exception all the molecules show only small deviations from planarity, the exception being the strongly carcinogenic 11-methyl-17-ketone in which the bay-region methyl group causes out-of-plane deformation of the benzo rings of 12.5 degrees. Among the other six compounds the two carcinogens are readily differentiated by high angle strain induced by a 7-methyl group or a 1,11-methano bridge. As expected, the HOMO's of these molecules to some extent reflect their ease of chemical oxidation at the 6,7-double bond; biological oxidation is less easy to correlate probably due to spatial restrictions at the active site within the mono-oxygenase.

Carcinogens↗
PubMed1982

The influence of various enzyme inducers on the microsomal metabolism of 7,8-benzoflavone.

Comparative in vitro metabolic experiments with 7,8-benzoflavone using microsomes prepared from the livers of control Sprague-Dawley rats or from rats of the same strain treated with phenobarbitone, 3-methylcholanthrene, or Aroclor 1254 (known inducers of hepatic cytochrome P450 mixed function oxidases) disclose variable amounts of the three metabolites previously reported, namely the 5,6-epoxide, 9,10-dihydro-9,10-diol, and the 5 (or 6)-phenol. Microsomes from phenobarbitone-treated rats gave in addition a small amount of the 5,6-dihydro-5,6-diol, identified by mass spectrometry and u.v. spectroscopy. The apparent discrepancy between our previous results (1) and those of Nesnow and Bergman (2) is therefore partly resolved, although the absence of the 9,10-diol from their products remains unexplained, but may have resulted from their use of rats of a different strain.

Animals↗
PubMed1982

High skin tumour initiating activity of the metabolically derived trans-3,4-dihydro-3,4-diol of the carcinogen 15,16-dihydro-11-methylcyclopenta[a]phenanthren-17-one.

Eight main metabolites of the parent carcinogen 15,16-dihydro-11-methylcyclopenta[a]phenanthrene-17-one (I) were assayed for their ability to initiate skin tumours in T.O. mice after topical application in two-stage experiments with croton oil used as the promoter. All were less active than 1 with the exception of the trans-3,4-dihydro-3,4-diol which was more than ten times as active. This diol is therefore confirmed as the proximate carcinogen, a conclusion reached previously on different evidence.

Animals↗
PubMed1982

Carcinogenicity and mutagenicity of some alkoxy cyclopenta[a]phenanthren-17-ones; effect of obstructing the bay region.

Six 11-O-alkoxy derivatives of the phenol 15,16-dihydro-11-hydroxycyclopenta[a]phenanthren-17-one were prepared and tested for their ability to initiate skin tumours on mouse skin after topical application, followed by promotion with croton oil. The 11-methoxy derivative was the most active, but was less so than the strong carcinogen 15,16-dihydro-11-methyl-cyclopenta[a]phenanthren-17-one (Ib). The 11-ethoxy derivative was somewhat less active as a tumour initiator than the 11-methoxy compound, and the 11-n-propoxy and 11-n-pentoxy derivatives were inactive. The 11-iso-propoxy and, surprisingly, the 11-n-butoxy compound possessed weak initiating activity. The phenol, which is too insoluble inorganic solvents to be tested in this way, proved to be about half as active as a skin tumour initiator compared with the 11-methyl derivative (Ib) when these compounds were injected s.c. in oil, and this was followed by topical promotion of dorsal skin remote from the site of injection. 15,16-Dihydro-15-methoxy-cyclopenta[a]phenanthren-17-one essentially lacked tumour initiating activity on mouse skin, but its 11-methyl homologue was moderately active. The carcinogenicity of the 11-alkoxy compounds was in general paralleled by their mutagenicity to Salmonella typhimurium TA100 in the Ames' test, with the exception of the 11-phenol which was not a mutagen under these conditions (plate assay).

Animals↗
PubMed1981

DNA adducts of the carcinogen, 15,16-dihydro-11-methylcyclopenta[a]phenanthren-17-one, in vivo and in vitro: high pressure liquid chromatographic separation and partial characterization.

The 11-methyl derivative (11-methyl ketone) is the most carcinogenic of the series of methylated derivatives based on 15,16-dihydro-cyclopenta[a]phenanthren-17-one. The nucleoside adducts derived from [3H]-11-methyl ketone-modified mouse skin DNA have been separated by both Sephadex LH-20 chromatography and reverse-phase h.p.l.c. and compared to those derived from DNA modified in vitro with the [14C]-11-methyl ketone using rat liver microsomes. The in vivo modified DNA separated to give 6 adducts (designated I--VI) on h.p.l.c. The major in vivo adduct (80% total adducts) co-chromatographed with the major in vitro adduct. The metabolites of the 11-methyl ketone (designated a--g) have been separated by h.p.l.c., and the adducts derived from each of these individual metabolites determined by further metabolism in the presence of DNA. H.p.l.c. separation of these adducts has allowed characterization of the in vivo adducts. The major adduct (V) and possibly one of the minor adducts (IV) were derived from the 3,4-dihydro-3,4-diol of the 11-methyl ketone (metabolite e). Adducts II and III were derived from the 16- and 15-monohydroxylated derivatives of the 11-methyl ketone and also from their corresponding 3,4-diols and therefore are likely to be the 16- and 15-hydroxy derivatives, respectively, of adduct V. Adduct VI, however, although derived from the 15-hydroxy-3,4-diol had a late retention time on h.p.l.c., suggesting either a non-diol-epoxide adduct or a deoxyadenosine adduct. The use of [3H-G]DNA has established that the major adduct (V) and the 16-hydroxy-derived adduct (II) contain deoxyguanosine. Reaction of the carcinogen with [3H-A]poly[dA-dT) gave adduct VI which was the only adduct peak shown to contain [3H]deoxyadenosine.

Animals↗
PubMed1981

Mass spectral characterisation of the major DNA--carcinogen adduct formed from the metabolically activated carcinogen 15,16-dihydro-11-methylcyclopenta[a]phenanthren-17-one.

E. coli DNA, labelled with [14C]adenine and [14C]-guanine, was allowed to react with the [3H]-labelled carcinogen 15,16-dihydro-11-methylcyclopenta[a]phenanthren-17-one in the presence of a microsomal metabolising system. Enzymatic hydrolysis of the DNA followed by Sephadex LH20 chromatography of its constituent nucleosides established that the major DNA - carcinogen adduct involved guanine, and not adenine. This was confirmed by submitting calf thymus DNA, which had been allowed to react with the unlabelled carcinogen, to pyrolysis electron impact mass spectrometry without further derivatisation. Analysis of a selected ion product (m/z 368) by means of mass-analysed kinetic energy spectrometry, a technique which allows study of the further fragmentation of the single, selected ion, revealed that the guanine moiety was attached via the nitrogen atom of its exocyclic amino group to C-1 of a 1,2,3,4-tetrahydro-2,3,4-trihydroxy derivative of the original carcinogen.

Biotransformation↗
PubMed1981

Co-carcinogenicity of promoting agents.

An experiment is described to investigate whether coadministration of a promoting agent would enhance the carcinogenicity of a repeatedly administered complete carcinogen. Topical application of the strong carcinogen 15,16-dihydro-11-methylcyclopenta[a]phenanthren-17-one (I) in toluene containing 1% v/v croton oil is about five times more effective than applications in toluene alone, as judged from the respective mean latent periods. A similar effect is also apparent for benzo[a]pyrene.

Animals↗
PubMed1980

Covalent binding of polycyclic aromatic compounds to mitochondrial and nuclear DNA.

Since the pioneering work of the Millers it has become clear that most chemical carcinogens require metabolism to reactive electrophiles and then exhibit their carcinogenic potential by reacting chemically with, and modifying, cellular macromolecules. At first modification of proteins was considered most likely to be of importance in carcinogenesis. Later, Brookes and Lawley demonstrated that the extent of binding of several polycyclic hydrocarbons to DNA, but not to RNA or protein isolated from the skin of mice treated topically with these compounds, correlated with their known carcinogenic potency to this tissue. Mammalian cells, particularly mouse embryo cells, treated with chemical carcinogens have often been used, and DNA has been involved almost exclusively from whole cells. However, mitochondria possess unique DNA which accounts for 0.1-1% of the total DNA present in mammalian cells, and three studies have shown that carcinogenic alkylating agents modify the michondrial DNA by a factor about five times greater than the nuclear DNA from the same cells. We demonstrate here that with six polycyclic aromatic compounds, all of which require metabolic activation and bind to DNA to a much smaller extent than direct than direct-acting alkylating agents, the binding to mitochondrial relative to DNA is dramatically increased by a factor of nearly 50 to over 500.

Animals↗