Nitroso transfer from alpha-nitrosamino aldehydes: implications for carcinogenesis.

Six alkylnitrosamino ethanols (R-N(NO)-CH2CH2OH; R = Me, nBu, sBu, iBu, tBu, HOCH2CH2-), including the potent carcinogen N-nitrosodiethanolamine, have been shown to undergo efficient liver alcohol dehydrogenase catalyzed oxidation to their corresponding alpha-nitrosamino aldehydes. Five structurally representative nitrosamino-ethanals (R-N(NO)CH2CHO, R = 4-ClC6H4-, CH3-, nBu-, tBu-, HOCH2CH2-) have been synthesized. Each of these compounds demonstrates the unusual property of facile transnitrosation to a secondary amine. Transnitrosation to dimethylamine, pyrrolidine, morpholine and N-methylaniline has been shown. This reaction occurs rapidly at room temperature in organic solvents but is somewhat slower in aqueous buffer due to extensive equilibrium formation of gem diols by hydration of the aldehyde group. In aqueous media the transnitrosation rate increases with increasing pH from 7 to 9 and does not occur at pH 4. Transnitrosation to primary amines results in deamination (benzylamine----benzyl alcohol). The transnitrosation reaction is accompanied by the formation of imines of glyoxal (R - N = CH - CH = N - R) which appear as primary amines and glyoxal in aqueous solution. Other products have also been characterized as well. These chemical and biochemical data, taken together with results in other laboratories, provide strong support for our hypothesis that certain beta-oxidized nitrosamines can be activated to proximate or ultimate carcinogens by biochemical oxidation to produce highly reactive nitrosamines.

Alpha-nitrosaminoaldehydes: highly reactive metabolites.

alpha-Nitrosamino aldehydes are highly reactive compounds which are directly-acting mutagens and are capable of facile transnitrosation to secondary and primary amines. The latter lead reactions to deamination. N-Nitrosobutyl(2-oxoethyl)amine (NBOEA) undergoes spontaneous decomposition in buffer at pH greater than 7 (25 degrees C) to give glyoxal and products implicating the formation of the butyl diazonium ion. NBOEA reacts with guanosine to produce xanthosine (by deamination), 7-butylguanosine and the 1,N2 glyoxal adduct, among other products. All beta-nitrosaminoethanols investigated undergo liver alcohol dehydrogenase-catalysed oxidation to their corresponding aldehydes. Several of these aldehydes have been shown to be directly-acting mutagens. These data provide strong evidence for an alternative carcinogenic bioactivation route for nitrosamines which does not involve alpha-oxidation.

Ester-mediated nitrosamine formation from nitrite and secondary or tertiary amines.

N-Nitrosamines are formed from the heating of either a secondary or a tertiary amine with sodium nitrite in the presence of a high-boiling ester such as 2-acetoxyethanol in ethylene glycol. The four secondary and six tertiary amines examined were found to produce N-nitrosamines in yields ranging from 4% to 80% when equimolar amounts of amine and ester were heated at 120 degrees C with one- to ten-fold equivalents of sodium nitrite in ethylene glycol. Secondary amines competitively produced acetamides at a rate slightly greater than N-nitrosamine formation. Preincubation of a large excess of sodium nitrite and ester led to the rapid formation of N-nitrosamines in high yield. The reaction of tribenzylamine resulted in the formation of both benzaldehyde and dibenzylnitrosamine. N,N-Dimethylbenzylamine reacted to give nearly equimolar amounts of N-nitrosodimethylamine and N-nitroso-N-methylbenzylamine. It is proposed that the nitrosating agent is a nitrous ester, and it is shown that 2-benzoxyethyl nitrite rapidly nitrosates secondary and tertiary amines under these reaction conditions. It is also proposed that these transformations are good models for the environmental formation of N-nitrosamines in foods and commercial products.

Alternative bioactivation routes for beta-hydroxynitrosamines. Biochemical and chemical model studies.

The biochemical retroaldol-like fragmentation of beta-hydroxynitrosamines has been investigated further. The extent of fragmentation of 2-hydroxy-2-methylpropyl-methylnitrosamine (HMPMN) to N-nitrosodimethylamine and acetone induced by metabolic activation increases as the NADPH level is decreased. 2-Hydroxy-2-phenylethyl-methylnitrosamine (HPhEMN) undergoes competitive oxidation to 2-oxy-2-phenylethyl-methylnitrosamine (OPhEMN) and fragmentation to benzaldehyde and N-nitroso-dimethylamine in the presence of a metabolic activation system from rat liver. The extent of the oxidation was increased by preinduction of the rats with phenobarbital, or separate addition of NADPH and NAD, but was decreased by addition of dimethyl sulfoxide. The fragmentation was observed most readily when oxidation was inhibited or was not induced by cofactors. When HPhEMN was administered to a rat intraperitoneally, benzaldehyde (fragmentation) was found in the urine with OPhEMN and the substrate, but only the last two substances were found in liver and blood. These experiments provide evidence for retroaldol-like fragmentation of beta-hydroxynitrosamines both in vitro and in vivo. In a related investigation, it was found that N-nitroso-N-4-chlorophenyl-2-aminoethanal (NCAE) is extremely reactive and induces spontaneous generation of 4-chlorobenzenediazonium ion in chloroform, as trapped by 2-naphthol. NCAE reacts with dimethylamine in chloroform, benzene or methanol to give N-nitrosodimethylamine and 4-chloroaniline, among other products. This suggests that beta-nitrosaminoaldehydes produced by the biooxidation of their corresponding alcohols could produce cell alteration through alkylation, deamination or transnitrosation.

Nitrosamine formation from ternary nitrogen compounds.

When amides of pyrrolidine are heated with sodium nitrite, small amounts of NPYR are formed. Both the rate of reaction and the nitrosamine yield are increased when ethylene glycol and glycerine are used as solvents. Kinetic evidence suggests that these polyols react with the amide to form esters and the corresponding amine. The esters then react with sodium nitrite to give nitrite esters which rapidly nitrosate the amine. The heating of LDEA with sodium nitrite gives relatively high yields of NDELA in relatively short times and this nitrosation can be explained by this mechanistic hypothesis. 2-N,N-dimethylaminomethylpyrrole reacts very rapidly with nitrous acid at 25 degrees to give NDMA as the sole nitrosamine. This suggests a new mechanism of tertiary amine nitrosation.