K-Region dihydrodiols

McMillan DC, PP Fu, CE Cerniglia (1987) Stereoselective fungal metabolism of 7,12-dimethylbenz[a]anthra cene identification and enantiomeric resolution of a K-region dihydrodiol. Appl Environ Microbiol 53 2560-2566. 

Sutherland JB, JP Freeman, AL Selby, PP Fu, DW Miller CE Cerniglia (1990) Stereoselective formation of a K-region dihydrodiol from phenanthrene by Streptomyces flavovirens. Arch Microbiol 154 260-266. 

The configuration of the 4R,5R-dihydrodiol was established by application of the exciton chirality method (6). To minimize undesired interactions between the electric transition dipoles of the two j>-N,N-dimethylaminobenzoate chromophores and the dihydrodiol chromo-phore, a 4,5-dihydrodiol enantiomer was first reduced to 1,2,3,3a,4,5,7,8,9,10-decahydro and 4,5,7,8,9,10,11,12-octahydro derivatives (6). We found that it is not necessary to reduce the chrysene chromophore of a BaP 4,5-dihydrodiol enantiomer (Figure 2). Similarly, the absolute configurations of the K-region dihydrodiol enantiomers of BA (7), 7-bromo-BA (8), 7-fluoro-BA (9), 7-methyl-BA (10). and 7,12-dime thy 1-BA (DMBA) (7 ) can also be determined by the exciton chirality method without further reduction. 

The formation of BaP 9,10-epoxide is considered a reaction of detoxification of BaP, as is the formation of its K-region epoxide. However, this last statement must now be qualified by the recent finding that the K-region dihydrodiol of benzo[a]pyrene, i.e., /ra .v-4,5-dihydro-4,5-dihydroxy-BaP (10.35, Fig. 10.13), can ultimately form DNA adducts. This is due to its further metabolism to bis [dihydrodiols] such as 1,2,4,5-, 4,5,7,8-, and 4,5,9,10-bis[dihydrodiol]. The tran,y-/ra ,y-BaP-4,5,7,8-bis[dihydrodiol] has been shown to be a metabolic intermediate in the formation of DNA-adducts [103],... 

Yang, S., Mushtaq, M., and Fu, P., Elution order absolute configuration of k-region dihydrodiol enantiomers of benz[a] anthracene derivatives in chiral stationary phase high performance liquid chromatography, J. Chromatogr., 371, 195, 1986. 

Figure 8. Synthesis of K-region arene oxides from cis and trans K-region dihydrodiols.

K. Pal, B. Tierney, P. L. Grover, and P. Sims, Induction of sister chromatid exchanges in Chinese hamster ovary cells treated in vitro with non-K-region dihydrodiols of 7-methylbenz(cz)anthracene and benzo(a)pyrene, Mutat. Res. 50, 367-375 (1978). 

Figure 3. Mechanism of microsomal EH-catalyzed hydration of the K-region epoxide enantiomers of BA, BaP, and DMBA. The percentages of the trans-addition product by water for each enantiomeric epoxide are indicated. The enantiomeric composition of the dihydrodiol enantiomers formed from the hydration of DMBA 5S,6R-epoxide was determined using 1 mg protein equivalent of liver microsomes from pheno-barbital-treated rats per ml of incubation mixture and this hydration reaction is highly dependent on the concentration of the microsomal EH (49). The epoxide enantiomer formed predominantly from the respective parent hydrocarbon by liver microsomes from 3-methylcho-lanthrene-treated rats is shown in the box.

Reactive Metabolites of PAHs. A wide variety of products have been identified as metabolites of PAHs. These include phenols, quinones, trans-dihydrodiols, epoxides and a variety of conjugates of these compounds. Simple epoxides, especially those of the K-region, were initially favored as being the active metabolites responsible for the covalent binding of PAH to DNA. Little direct experimental support exists for this idea (62.63,64) except in microsomal incubations using preparation in which oxidations at the K-region are favored (65,66). Evidence has been presented that a 9-hydroxyB[a]P 4,5-oxide may account for some of the adducts observed in vivo (67.68) although these products have never been fully characterized. 

Detailed kinetic studies comparing the chemical reactivity ofK-region vs. non-K-region arene oxides have yielded important information. In aqueous solution, the non-K-region epoxides of phenanthrene (the 1,2-oxide and 3,4-oxides) yielded exclusively phenols (the 1-phenol and 4-phenol, respectively, as major products) in an acid-catalyzed reaction, as do epoxides of lower arenes (Fig. 10.1). In contrast, the K-region epoxide (i.e., phenanthrene 9,10-oxide 10.29) gave at pH < 7 the 9-phenol and the 9,10-dihydro-9,10-diol (predominantly trans) in a ratio of ca. 3 1. Under these conditions, the formation of this dihydrodiol was found to result from trapping of the carbonium ion by H20 (Fig. 10.11, Pathway a). At pH > 9, the product formed was nearly ex-... 

Initially, particular attention was focused on the epoxides of the so-called K region. As in the case of benzo [a] pyrene and certain other polycyclic aromatic hydrocarbons, these were more carcinogenic than the parent compound. The K region had attracted particular interest, as it is electronically the most reactive portion of the polycyclic aromatic hydrocarbon molecule. However, with other carcinogenic polycyclic aromatic hydrocarbons, this was not found to be the case. It now seems that the ultimate carcinogen is an epoxide of a dihydrodiol metabolite, where the epoxide is adjacent to the so-called bay region (Fig. 7.2). 

An important feature of K-region arene oxides, which is not shared by the unsubstituted non-K-region oxides, is their susceptibility to nucleophilic attack and production of dihydrodiols. When there are electron-withdrawing substituents on the non-K-region arene oxides, their behavior is also similar to that of the K-region arene oxides. Later work has shown that in the study of 1 reported above, the chloride ions used in the medium were taking part, and... 

K-region epoxide adducts at higher doses Dihydrodiol epoxide adducts at lower doses in most cases, but always accompanied by other adducts... 

Reduction of ortho-fDinpnes. Conventional reduction of non-K-region o-quinones such as 1 with LiAlHt affords the corresponding dihydrodiols in very low yield. Kundu has found that reduction of the dibromo derivatives of these quinones directly yields mixtures of cis- and Irani-dihydrodiols. 

In general, K-region arene oxides behave rather like aliphatic epoxides and thus readily undergo hydration reactions, whereas benzene oxides and non-K-region arene oxides form dihydrodiols much more reluctantly. Kinetic studies of the mechanism of solvolysis of phenanthrene 9,10-oxide 2 have been carried out in several laboratories.Below pH 7 the hydrolysis reaction was acid-catalyzed and the products included the trans- and c/s-9,10-dihydrodiols along with a preponderance of 9-phenanthrol, while above pH 7 the reaction proceeded via the spontaneous mechanism ( o) mainly the frans-dihydrodiol. 

Hydrolytic studies with other K-region arene oxides 3, 20, 4, and 109 have been reported. ° ° A comparative investigation of the mechanism of the solvolysis and rearrangement of K- and non-K-region arene oxides showed that dihydrodiols were not produced from non-K-region arene oxides and the exclusively formed phenols resulted mainly from the vinylogous benzylic carbonium ions. 

Configurational assignments for known K-region derivatives are given in Figure 19. In contrast to the benzo-ring franx-(R,R)-dihydrodiols, the K-region trans-... 

Figuie 19. Absolute configurations and signs of rotation (tetrahydrofuran) for resolved K-region derivatives of polycyclic aromatic hydrocarbons. References are superscripted. Rotation of the free cis-4,5-dihydrodiol of benzo[a]pyrenc was not determined because of facile autoxidation which resulted in colored solutions. With liver microsomal enzymes from 3-methylcholanthrene-treated rats, the frar7s-(R,R)-dihydrodiol greatly predominates (96%) from benzo[a]pyrene, predominates (68%) from benz[a]anthracene, appears to be the minor enantiomer from 12-methylbenz[a]anthracene, and is the minor enantiomer (42%) from phenanthrene. ... 

TABLE 3. ABSOLUTE CONFIGURATIONS AND SIGNS OF ROTATION ([a]D> FOR ENANTIOMERICALLY PURE, NON-K-REGION DERIVATIVES OF THE PAHS. The enantiomers shown for the arene oxides and dihydrodiols are either known or are expected to be those which predominate by the combined action of cytochrome P450c and epoxide hydrolase. The tetrahydrodiols result either by reduction of the dihydrodiols or by trans hydrolysis of the tetrahydroepoxides in which attack by water occurs at the benzylic oxirane carbon. ... 

Since it is the smallest aromatic hydrocarbon to have a bay-region and a K-region , phenanthrene is often used as a model substrate for studies on metabolism of carcinogenic PAHs. Phenanthrene is absorbed following oral and dermal exposure. Data from structurally related PAHs suggest that phenanthrene would be absorbed from the lungs. Metabolites of phenanthrene identified in in vivo and in vitro studies indicate that metabolism proceeds by epoxidation at the 1-2, 3-4, and 9-10 carbons, with dihydrodiols as the primary metabolites. 

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