Microsome-Mediated Oxidation of N-Nitrosodiethanolamine (NDELA), a Bident Carcinogen

Abstract

N-Nitrosodiethanolamine (NDELA), an environmentally prevalent, potent carcinogen, undergoes competitive rat liver microsome-mediated oxidation at both the alpha (adjacent to N)- and beta-positions of the 2-hydroxyethyl chains. The former process, alpha-hydroxylation, is detected by the formation of glycolaldehyde (determined as its 2,4-dinitrophenylhydrazone DNP) that is assumed to arise from the decomposition of the corresponding alpha-hydroxynitrosamine, which is also the progenitor of the 2-hydroxyethyldiazonium ion. This finding refutes prior published work that states that the alpha-hydroxylation of NDELA does not occur. Competitive microsomal oxidation at the beta-position gives the hemiacetal N-nitroso-2-hydroxymorpholine (NHMOR) at a rate 1.5 times alpha-hydroxylation. Glycolaldehyde is oxidized in this system to glyoxal at a rate 39 times the conversion of NDELA to glycolaldehyde. The alpha-hydroxylation of NHMOR at either C-3 or C-5 to give glyoxal or glycolaldehyde, respectively, occurs at respective rates 3-6 times that of the alpha-hydroxylation of NDELA. Ethylene glycol, a hydrolysis product of the 2-hydroxyethyldiazonium ion is shown to undergo microsome mediate oxidation to glyoxal. Ethyl-2-hydroxyethylnitrosamine (NEELA) undergoes a similar set of microsome-mediated oxidations at alpha-position of the ethyl (fastest) and 2-hydroxyethyl groups, as well as beta-oxidation of the 2-hydroxyethyl group, a process which is slightly more rapid than alpha-hydroxylation of the same chain. Comparisons of oxidations rates of these substrates, as manipulated by preinducers, isoniazid, streptozocin, and phenobarbital, and enzyme inhibitors diethyldithiocarbamate and 4-methylpyrazole, with that of dimethylnitrosamine, a substrate for cytochrome P450 2E1, strongly suggest that this isozyme is also responsible for the oxidations reported here. alpha-Deuteration of NDELA practically eliminates its alpha-hydroxylation by microsomes from isoniazid induced rats, but doubles beta-oxidation, while beta-deuteration of this substrate significantly reduces beta-oxidation and enhances alpha-hydroxylation. Since both glyoxal-guanine and 2-hydroxyethyl-DNA base adducts are known to arise from the in vivo administration of NDELA and because this work demonstrates that these two fragments can come from the microsomal oxidation of a single nitrosamine molecule containing the 2-hydroxyethyl group, NDELA and related nitrosamines are bident (two-toothed) carcinogens, a process which is likely to enhance their carcinogenic potency.

Automatic extraction of structural alerts for predicting chromosome aberrations of organic compounds

“…NDELA is a bident carcinogen that undergoes competitive rat liver microsome-mediated oxidation at both the a (adjacent to N)-and b-positions of the 2-hydroxyl chains as illustrated in Scheme 1 [38,39]. These are the two positions ''alerted'' by the bond contributions given below.…”

“…These are the two positions ''alerted'' by the bond contributions given below. NHMOR (N-nitrosoethylethanalamine) (see Scheme 1) has been observed to generate glyoxal-deoxyguanosine in DNA both in vitro and in vivo [39].…”

Stereospecific Deuterium Substitution Attenuates the Tumorigenicity and Metabolism of the Tobacco-Specific Nitrosamine 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)

“…Deuterium isotope effects have been used extensively in the study of the mechanisms of nitrosamine-induced carcinogenesis (18,(65)(66)(67)(68)(69)(70)(71)(72)(73)(74)(75)(76)(77)(78)(79)(80)(81)(82). The first experiment to utilize deuterium substitution in the study of nitrosamineinduced cancer demonstrated that perdeuteration of NDMA lowered the liver carcinogenicity of this compound in rats (65).…”

“…The first experiment to utilize deuterium substitution in the study of nitrosamineinduced cancer demonstrated that perdeuteration of NDMA lowered the liver carcinogenicity of this compound in rats (65). Subsequent work with NDMA and other nitrosamines showed that biological and KIEs could also be observed on levels of DNA adducts (18,73,78,81) and rates of metabolism, respectively (66,69,71,73,74,77,79,82). In the metabolism work, however, several conflicting results have been obtained.…”

Identification of Adducts Formed in the Reaction of α-Acetoxy-N-nitrosopyrrolidine with Deoxyribonucleosides and DNA

“…This is in contrast to acyclic nitrosamines such as N-nitrosodiethylamine which produce separate aldehyde and diazohydroxide products upon α-hydroxylation. The presence of two electrophilic centers in a molecule such as 4 results in a more complex pattern of DNA adduct formation than observed with acyclic nitrosamines, and is reminiscent of the bident carcinogens described by Loeppky, although in that case adduct formation results from two separate electrophiles generated from the same molecule (47). Thirteen dGuo adducts of α-acetoxyNPYR have been previously identified, and some of these result, directly or indirectly, from reactions at both electrophilic centers (31).…”