It has been demonstrated that in humans certain factors such as early menarche, late pregnancy, and nulliparity are associated with a higher risk of developing breast cancer, while early pregnancy acts as a protective factor. Induction of mammary cancer in rats by administration of the chemical carcinogen 7, 12-dimethylbenz(a)anthracene reveals that the same factors influencing human breast cancer risk also affect the susceptibility of the rat mammary gland to the chemical carcinogen. Nulliparous rats and rats undergoing pregnancy interruption are more susceptible to developing carcinomas. This fact has been attributed to the incomplete differentiation of the gland at the time of carcinogen administration. Parous rats are resistant to the carcinogenic effect of DMBA, which is explained by the complete development of the gland attained during pregnancy and lactation. This development is manifested by the differentiation of terminal end buds into secretory units, which have a smaller proliferative compartment; the epithelial cells of these secretory units have a longer cell cycle, less avidity for binding DMBA, and possess a more efficient DNA excision repair capacity.

Differentiation of the mammary gland and susceptibility to carcinogenesis

The Walden memorial at the Technical University in Riga is pictured in the frontispiece to mark the recent centennial of the Walden inversion. This is a rare public monument to key events from the first era of exploration in stereocontrolled synthesis, and may be the only such monument to use the language of organic chemistry expressed at the molecular level. The reaction of racemic substrates with chiral nucleophiles is one of many methods currently known to achieve kinetic resolution, a phenomenon that ranks as the oldest and most general approach for the synthesis of highly enantioenriched substances. The first nonenzymatic kinetic resolutions as well as the original forms of the Walden inversion were studied in the 1890s. All of these investigations were conducted within the first generation following the demonstration that carbon is tetrahedral, and provided abundant evidence that the principles and importance of enantiocontrolled syntheses were understood. However, a reliable, rapid technique to quantify results and guide the optimization process was still lacking. Many decades passed before this problem was solved by the advent of HPLC and GLPC assays on chiral supports, which stimulated explosive growth in the synthesis of nonracemic substances by kinetic resolution. The Walden monument is accessible to passers-by for hands-on inspection as well as for contemplation and learning. In a similar way, kinetic resolution is experimentally accessible and can be thought-provoking at several levels. We follow the story of kinetic resolution from the early discoveries through fascinating historical milestones and conceptual developments, and close with a focus on modern techniques that maximize efficiency.

Efficiency in nonenzymatic kinetic resolution.

Publisher Summary This chapter deals with the microbial transformation of polycyclic aromatic hydrocarbons (PAHs) The similarities and differences between the microbial and mammalian metabolism are described Bacteria, filamentous fungi, yeasts, cyanobacteria, diatoms, and other eukaryotic algae have the enzymatic capacity to oxidize PAHs that range in size from naphthalene to benzo[ a ]pyrene The hydroxylation of PAHs always involves the incorporation of molecular oxygen; however, there are differences in the mechanism of hydroxylation of PAHs by prokaryotic and eukaryotic microorganisms Bacteria oxygenate PAHs to form a dihydrodiol with a cis configuration The genes for the initial oxidation of PAHs are localized on plasmids In contrast to bacteria, fungi oxidize PAHs via a cytochrome P-450 monooxygenase to form arene oxide, which can isomerize to phenols or undergo enzymatic hydration to yield trans-dihydrodiols Multiple oxidative pathways may be involved in the cyanobacterial metabolism of PAHs Studies on PAH metabolism are entering a new era; biochemical genetic techniques such as gene cloning and transposon mutagenesis will provide new insight into the biochemistry and regulation of PAH degradative pathways

Microbial metabolism of polycyclic aromatic hydrocarbons.

Synthetic antioxidants: biochemical actions and interference with radiation, toxic compounds, chemical mutagens and chemical carcinogens

Das im Vortitel gezeigte Walden-Denkmal auf dem Campus der Technischen Universitat Riga ist sicher eines der wenigen offentlichen Monumente, die Schlusselereignisse aus der Anfangszeit der stereokontrollierten Synthese thematisieren und vielleicht das einzige, das die “molekulare Sprache” der organischen Chemie gebraucht. Die ersten nichtenzymatischen kinetischen Racematspaltungen wurden, ebenso wie die ursprunglichen Varianten der Walden-Umkehr, in den 1890er Jahren untersucht – also noch in der ersten Forschergeneration nach Entdeckung der tetraedrischen Natur des Kohlenstoffs. Aus diesen fruhen Arbeiten ist abzulesen, dass die Prinzipien und die Bedeutung enantiokontrollierter Synthesen bereits gut verstanden waren. Was noch fehlte war eine verlassliche und schnelle Methode zur Quantifizierung der Ergebnisse und zur Optimierung der Experimente. Viele Jahrzehnte sollten vergehen, bis das Problem mit Einfuhrung von HPLC- und GPLC-Testreihen auf chiralen Tragern gelost wurde. Das Walden-Denkmal ladt den Vorubergehenden zur spielerischen Betrachtung ein – auf ahnliche Weise, wie auch die kinetische Racematspaltung zum Experimentieren einladt. Dieser Aufsatz verfolgt den Verlauf der Geschichte angefangen von den ersten Entdeckungen uber die faszinierenden historischen Meilensteine und konzeptionellen Entwicklungen und schliest mit einem Blick auf moderne Techniken zur Effizienzmaximierung.

Effizienz in der nichtenzymatischen kinetischen Racematspaltung

Thirty-nine derivatives of 7,12-dimethylbenz(a)anthracene (DMBA) were tested for mutagenicity towards Salmonella typhimurium strain TA100 in the presence of activating enzymes. The compounds tested included four sets of 3,4-, 5,6-, 8,9-, and 10,11-diols: those of DMBA, its 7- and 12-hydroxymethyl derivatives, and the 7,12-dihydroxymethyl derivative. Several phenolic derivatives and formyl and carboxylic acid derivatives at the 7 and 12 positions of DMBA were also tested. At the concentrations examined, only the 3,4-diols of DMBA and 7hydroxymethyl-12-methylbenzanthracene (7-OHM-12-MBA) were strongly mutagenic. These two 3,4-dihydrodiols were of equal mutagenicity. When DMBA, its three hydroxymethyl derivatives, and their respective dihydrodiols were tested for tumor-initiating activity in the mouse skin two-stage tumorigenesis model, the 3,4-diol of DMBA was the most potent initiator, causing a tumor inci dence after 23 weeks of promotion of 9.8 papillomas/mouse at a dose of 3 nmol/mouse. At this same dose level, the equally mutagenic 3,4-diol of 7-OHM-12-MBA caused a tumor inci dence of only 0.9 papillomas/mouse, while 7-OHM-1 2-MBA caused only 0.26 papillomas/mouse. The other metabolites tested were less tumorigenic than DMBA, which caused a tumor incidence of 2.9 papillomas/mouse at a dose of 3 nmol/ mouse. Therefore, the 3,4-diol of DMBA qualifies as a proxi mate carcinogen of DMBA and indicates that the bay-region 3,4-diol-1,2-epoxide of DMBA is probably an ultimate carcin ogen of DMBA in mouse skin. The 3,4-diol of 7-OHM-12-MBA, although not a proximate carcinogen of DMBA, appears to be a proximate carcinogen of 7-OHM-12-MBA and may therefore contribute to some of the carcinogenicity observed with DMBA through this hydroxymethyl derivative. The data also indicate the lack of a quantitative agreement between mutagenicity and carcinogenicity under the conditions of the tests used.

https://cancerres.aacrjournals.org/content/canres/40/10/3661.full.pdf

Carcinogenicity and Mutagenicity of the 3,4-Dihydrodiols and Other Metabolites of 7,12-Dimethylbenz(a)anthracene and Its Hydroxymethyl Derivatives

Detection, hepatotoxicity, and tumorigenicity of pyrrolizidine alkaloids in Chinese herbal plants and herbal dietary supplements

The metabolism of 7,12-[14C]dimethylbenz(a)anthracene ([14C]DMBA) and 7-[7-CH2-3H]hydroxymethyl-12-methylbenz(a)anthracene ([7-CH2-3H]7-OHM-12-BMA) by rat liver nuclei and microsomes was studied by high-performance liquid chromatography. DMBA and 7-OHM-12-MBA are metabolized at methyl group(s) and at the aromatic ring carbons to form trans-dihyrodiols at positions 3, 4, 5, 6, 8, 9, and 10,11 and phenols at positions 2, 3, and 4 by both nuclear and microsomal enzymes. Both nuclear and microsomal monooxygenase enzyme activities were inducible by pretreatment of the animals with phenobarbital or 3-methylcholanthrene. The rates of formation of all metabolites by microsomes were five- to 20-fold higher than those by nuclei in metabolizing DMBA or 7-OHM-12-MBA. The presence of a hydroxyl group at the 7-methyl position of DMBA markedly decreased the rate of metabolism. The rate of total metabolism of 7-OHM-12-MBA was only 20 to 70% of that of DMBA under identical in vitro incubation conditions. The 3-methylcholanthrene- and phenobarbital-induced enzymes showed a significantly different regioselectivity toward the metabolism of DMBA or 7-OHM-12-MBA and are attributed to different forms of cytochrome P-450 present in the enzyme preparations.

https://cancerres.aacrjournals.org/content/canres/41/4/1559.full.pdf

Metabolism of 7,12-dimethylbenz(a)anthracene and 7-hydroxymethyl-12-methylbenz(a)anthracene by rat liver and microsomes.

The newborn mouse lung adenoma model has been shown to be a sensitive test for studying the tumorigenicity of bay region diol epoxides and their precursor dihydrodiols. When a total dose of 28 nmol of 7,12-dimethylbenz[a]anthracene (DMBA) or its derivatives was injected i.p. into the preweaning mice, it was found that the 3,4-dihydrodiols of both DMBA and 7-hydroxymethyl-12-methylbenz[a]anthracene caused 13.3 and 4.1 times more lung adenomas than DMBA, respectively. The mice treated with the 5,6- and 8,9-dihydrodiols of DMBA, 7-hydroxymethyl-12-methylbenz[a]anthracene and its 5,6- and 8,9- and 10,11-dihydrodiols, 7-methyl-12-hydroxymethylbenz[a]anthracene and 7,12-dihydroxymethylbenz[a]anthracene developed a level of lung adenomas/mouse less than 2-fold higher than that found in the DMSO-treated control group. Liver tumors also developed in some of the mice. The percentage of mice with liver tumors also indicated that the 3,4-dihydrodiols of both DMBA and 7-hydroxymethyl-12-methylbenz[a]anthracene were more tumorigenic than DMBA itself. These data indicate that the 3,4-dihydrodiols of both DMBA and its 7-hydroxymethyl derivative may be proximate carcinogenic metabolites of DMBA in the newborn mouse.

Tumorigenicity of 7, 12-dimethylbenz[a]anthracene, its hydroxy-methylated derivatives and selected dihydrodiols in the newborn mouse

The effects of butylated hydroxyanisole (BHA) on the metabolism of benzo[a]pyrene (BP) were investigated with mouse hepatic microsomes. Microsomes from BHA-fed mice showed a higher activity in catalyzing the formation of most metabolites from BP but a much lower activity in the formation of 9-hydroxy BP than control microsomes. Dietary BHA treatment enhanced the total metabolism of BP but decreased the microsomal metabolism of BP-7,8-diol, especially the formation of BP-trans-7,8-diol-anti-9,10-oxide. The altered metabolism of BP is believed to be due to the induction of new cytochrome P-450 species by dietary BHA. Consistent with this interpretation is the observation that the treatment also decreased the Km and increased the Vmax of ethoxycoumarin O-dealkylase. Short-term treatment (BHA administered p.o.) and in vitro added BHA were shown to inhibit BP metabolism. Thus, BHA can affect BP metabolism by exerting its inhibitory effect directly and by altering the composition of microsomal monooxygenase enzymes after a few days of exposure.

Ming W. Chou

Papers

Carcinogenicity and Mutagenicity of the 3,4-Dihydrodiols and Other Metabolites of 7,12-Dimethylbenz(a)anthracene and Its Hydroxymethyl Derivatives

Detection, hepatotoxicity, and tumorigenicity of pyrrolizidine alkaloids in Chinese herbal plants and herbal dietary supplements

Metabolism of 7,12-dimethylbenz(a)anthracene and 7-hydroxymethyl-12-methylbenz(a)anthracene by rat liver and microsomes.

Tumorigenicity of 7, 12-dimethylbenz[a]anthracene, its hydroxy-methylated derivatives and selected dihydrodiols in the newborn mouse

Regioselective inhibition of benzo[a]pyrene metabolism by butylated hydroxyanisole