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.