Nitrosation of aspartic acid, aspartame, and glycine ethylester. Alkylation of 4-(p-nitrobenzyl)pyridine (NBP) in vitro and binding to DNA in the rat.
In a colorimetric assay using 4-(p-nitrobenzyl)pyridine (NBP) as a nucleophilic scavenger of alkylating agents, the nitrosation and alkylation reactions were investigated for a number of amino acids and derivatives. The alkylating activity increased with the square of the nitrite concentration. The nitrosation rate constants for aspartic acid, aspartame, and glycine ethylester (= precursors C) were 0.08, 1.4 and less than or equal to 0.2, respectively, expressed in terms of the pH-dependent k2 rate constant of the equation dNOC/dt = k2.[C].[nitrite]2. The rates correlated inversely with the basicity of the amino group. The stability of the alkylating activity was astonishingly high, both in acid and at neutral pH. Half-lives of 500, 200, and 30 min were determined for aspartic acid (pH 3.5), aspartame (pH 2.5), and glycine ethylester (pH 2.5). Values of 60, 15, and 2 min, respectively, were found at pH 7. It is concluded that rearrangement of the primary N-nitroso product to the ultimate alkylating agent could be rate-limiting. The potential of nitrosated alpha-amino acids to bind to DNA in vivo was investigated by oral gavage of radiolabelled glycine ethylester to rats, followed immediately by sodium nitrite. DNA was isolated from stomach and liver and analysed for radioactivity and modified nucleotides. No indication of DNA adduct formation was obtained. Based on an estimation of the dose fraction converted from glycine ethylester to the nitroso product under the given experimental conditions, the maximum possible DNA-binding potency of nitroso glycine ethylester is about one order of magnitude below the methylating potency of N-nitrosomethylurea in rat stomach. The apparent discrepancy to the in vitro data could be due to efficient detoxification processes in mammalian cells.
Meier
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Institute of Toxicology, Swiss Federal Institute of Technology, University of Zurich.
Shephard
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SE
Lutz
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WK
D000478:Alkylation; D000818:Animals; D001218:Aspartame / Q000378:metabolism*; D001224:Aspartic Acid / Q000378:metabolism*; D004247:DNA / Q000378:metabolism*; D004249:DNA Damage; D004151:Dipeptides / Q000378:metabolism*; D005998:Glycine / Q000031:analogs & derivatives* / Q000378:metabolism; D006207:Half-Life; D008297:Male; D009573:Nitrites / Q000378:metabolism; D015538:Nitrosation; D011725:Pyridines / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains
D004151:Dipeptides; D009573:Nitrites; D011725:Pyridines; C008633:4-(4-nitrobenzyl)pyridine; D001224:Aspartic Acid; C022010:glycine ethyl ester; D004247:DNA; D005998:Glycine; D001218:Aspartame