Chapter 33
Chemistry of Putative Intermediates in Bioactivation ofβ-OxidizedNitrosamines Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 29, 2018 | https://pubs.acs.org Publication Date: March 28, 1994 | doi: 10.1021/bk-1994-0553.ch033
Richard N. Loeppky, Eric Erb, Aloka Srinivasan, and Li Yu Department of Chemistry, University of Missouri, Columbia, MO 65211
1,2,3-Oxadiazolinium cations 1 and 2 have been proposed as intermediates in the bioactivation of β-hydroxynitrosamines. Enzymatic sulfation of the OH of methylethanolnitrosamine 3a followed by cyclization to 1 has been proposed as mechanism for the in vivo methylation of DNA by 3a (1). 3-Alkyl-5-hydroxy-1,2,3-oxadiazolinium cations 2 (R =H) form from α-nitrosamino aldehydes upon pro tonation (2), and may be intermediates in the transni trosation reactions of these aldehydes which are formed enzymatically from the oxidation of their corresponding alcohols (3). A goal of this research is the elucidation of the poorly understood chemistry of 1,2,3oxadiazolinium cations toward nucleophiles of biological significance. 3-Alkyl-5-hydroxy-1,2,3-oxadiazolinium cations are too unstable for de finitive studies with bases and nucleophiles. Their 5-methoxy analogs 2 (R =CH ) were prepared by treatment of the methyl acetals of the respective α-nitrosamino aldehydes with trimethylsilyl triflate. The t of 1 in water at 25° C is 5 days, (3a is the exclusive product) whereas that of 2 (R,R =CH ) is less than 5 minutes. The reactions of these compounds were studied in aqueous as well as organic solvents. In order to determine whether oxadiazolinium cations of structure type 2 could be intermediates in transnitrosation reactions of α-nitrosamino aldehydes, we examined the reactions of 2 (R = CH3 or C6H5 Ri=CH3) with morpholine. N nitrosomorpholine (NMOR) forms from both substrates in CH2CI2 (CH3,18%; C6H5,54%). Reaction of 2 (R = CH3) with morpholine in CH2CI2 also gener ates a 1% yield of N-methylmorpholine. Attack of morpholine at the O-CH3 of 2 would yield methylethanalnitrosamine which is known to transnitrosate morpholine. But no N-methylmorpholine is found among the reaction products of 2 (R=CgH5, Rl=CH3) ruling out this pathway. These results suggest a possible role for 3-alkyl-5-hydroxy-l,2,3-oxadiazolinium cations in the interesting transnitrosation chemistry of α-nitrosamino aldehydes. 1
1
1/2
1
3
0097-6156/94/0553-0334$08.00A) © 1994 American Chemical Society
Loeppky and Michejda; Nitrosamines and Related N-Nitroso Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
3
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 29, 2018 | https://pubs.acs.org Publication Date: March 28, 1994 | doi: 10.1021/bk-1994-0553.ch033
33. LOEPPKY ET AL.
Bioactivation of β-Ondized Nitrosamines
335
The reactions of 1 and 2 toward nucleophiles are complicated because the protons adjacent to the positively charged Ν are acidic enough to be easily abstracted at pH 9-10 (4). Basic nucleophiles attack as both nucleophiles and bases. Morpholine does not react with 1 in either H2O or CH2CI2 to produce NMOR. In CH2CI2 it attacks C-5 to give a 25% yield of the open chained nitrosamine 3b and attack at the methyl group to give N-methylmorpholine is relatively minor (13%). We have also shown that thiophenol (in CH2CI2 , but not H2O) attacks C-5 to give 3c (33%). This is the only carbon attacked by water (giving 3a), and Michejda has also shown that it is the primary position for attack by acetic acid and aniline (5). Guanine derivatives and guanine residues in DNA react with 1 to incorporate the entire nitrosamine fragment at (3d). This is the major reaction of 1 with DNA in vitro. Only small amounts of base methylation are observed. Despite numerous experiments designed to specifi cally detect the products of nucleophilic attack at C-4 in 1 and related compounds we have not observed any attack at this position. The major product of the reaction of 1 with PhSH in H2O is diphenyldisulfide, PhS-SPh (92%) the other isolated products were PhSCH3 ( 5%) and 3c (1%). In CH2CI2 the yield of PhS-SPh is decreased (13%) and the products of nu cleophilic substitution are increased (47%). Because of the difficulty of product isolation with 1 the reaction of the analogous 3-phenyl-l,2,3-oxadiazolinium triflate with PhSH was examined. The yield of PhS-SPh in aqueous buffer at pH 7.4. was 41%. Other key products included benzene (60%), azobenzene (8%), and glycolaldehyde (75%). These products and the observations of the Hunig group (4) suggest the involvement of phenyldiazene (Ph-N=NH) as a reaction intermediate. Generation of this unstable compound from potassium phenyldiazocarboxylate in the presence of PhSH also led to the disulfide and many of the other reaction products produced from - Ά the reaction of R H Man. 2 (R=C H , :B Rl=CH3) with PhSH PhSH. Further work showed 2 R H + PhS-SPh + HScheme that the reaction of 1 with morpholine in H2O generated significant quantities of methane, which we believe is formed from methyldiazene. We believe that these compounds are arising through abstraction of the C-4 H of proton of oxadiazolinium cation as shown in the scheme. The oxidation rate of N-acetylcysteine by 1 in aqueous buffer occurs most rapidly at higher pH's. It is probable that base generates the diazene which oxidizes the thiol. If 1 were generated in vivo its reduction by abundant thiols is very likely. Collectively, these data cast doubt on the ability of 1 to methylate DNA in vivo. 6
5
+ H,
2
OH R^SM tI H HCCH
Loeppky and Michejda; Nitrosamines and Related N-Nitroso Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 29, 2018 | https://pubs.acs.org Publication Date: March 28, 1994 | doi: 10.1021/bk-1994-0553.ch033
336
NITROSAMINES AND RELATED JV-NTTROSO COMPOUNDS
Acknowledgment: The support of this research by grant RO1-ES03953 from the National Institute of Environmental Health Sciences is gratefully acknowledged. References: (1) Kroeger-Koepke, M . B.; Koepke, S. R.; Hernandez, L . ; Michejda, C. J. Cancer Res. 1992, 52, 3300-3305 and references contained therein. (2) Loeppky, R. N . ; Fleischmann, E. D.; Adams, J. E.; Tomasik, W.; Schlemper, E. O.; Wong, T. C. J. Am. Chem. Soc. 1988,110, 5946-5951. (3) Loeppky, R. N . ; Tomasik, W.; Kerrick, Β., E. Carcinogenesis (London) 1987, 8, 941-946. (4) Hünig, S. Helv. Chim. Acta 1971, 54, 1721-1747.. (5) Michejda, C. J.; Koepke, S. R.; Kupper, R. IARC Sci. Publ.,1980, 31 (NNitroso Compd.: Anal., Form. Occurrence), 155-67. RECEIVED November 15, 1993
Loeppky and Michejda; Nitrosamines and Related N-Nitroso Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
