The Carcinogenic Significance of Reactive Intermediates Derived from 3-Acetoxy- and 5-Acetoxy-2-hydroxy-N-nitrosomorpholine

• Published in: Chemical research in toxicology Vol. 18; no. 12; pp. 1955 - 1966
• Main Authors: Loeppky, Richard N, Sukhtankar, Sunil, Gu, Feng, Park, Misun
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 01.12.2005
• Subjects: Acetates - chemistry; Carcinogens - chemistry; Hydrolysis; Isomerism; Kinetics; Molecular Structure; Morpholines - chemistry; Nitrosamines - chemistry; Nitroso Compounds - chemistry; Reactive Oxygen Species - chemistry
• ISSN: 0893-228X; 1520-5010
• DOI: 10.1021/tx0502037

N-Nitroso-2-hydroxymorpholine (NHMOR), a relatively reactive metabolite of two potent carcinogens, N-nitrosodiethanolamine (NDELA) and N-nitrosomorpholine (NMOR), has been reported to not be carcinogenic. Two isomeric acetate esters of the α-hydroxynitrosamines expected to be produced from the cytochrome P450-mediated metabolism of NHMOR have been synthesized, and their hydrolytic decomposition products, hydrolysis rates, and deoxyguanosine (dG) reaction adducts have been determined. N-Nitroso-3-acetoxy-2-hydroxymorpholine was prepared in high yield from the reaction of N-nitroso-2,3-dehydromorpholine with dry peracetic acid in glacial acetic acid or by the reaction of its dimethyldioxirane-produced epoxide with glacial acetic acid. The hydrolysis of this α-acetoxynitrosamine gave acetaldehyde (10%), ethylene glycol (55%), glyoxal (95%), and acetic acid. The pH rate profile for the hydrolysis of this nitrosamine was abnormal in that it exhibited pronounced base-catalyzed hydrolysis beginning at pH 5. The mechanism of hydrolytic decomposition is proposed to involve neighboring group participation with the formation of a reactive epoxide intermediate. N-Nitroso-3-acetoxy-2-hydroxymorpholine reacted with dG to give these guanine adducts after acidic deglycosylation:  1,N 2-glyoxal (65%), 7-(2-hydroxyethyl)guanine (9%), and O6-hydroxyethylguanine (3%). N-Nitroso-5-acetoxy-2-hydroxymorpholine was synthesized from 2-hydroxyethylvinylnitrosamine by its oxidative conversion to the corresponding aldehyde followed by reaction with dry peracetic acid in glacial acetic. The hydrolytic decomposition products of this nitrosamine were 2-acetoxyacetaldehyde (65%), a rearrangement product, glycol aldehyde (15%), a trace of glyoxal, and acetic acid. The pH rate profile for the hydrolysis of this acetate is similar to other α-acetoxynitrosamines in that it exhibits a pH-independent region which gives way to base-catalyzed ester hydrolysis beginning at pH 7. The lower pH (≈ 7 < 9) onset of base catalysis is proposed to involve base-catalyzed opening of the hemiacetal and intramolecular acyl transfer to give an unstable α-hydroxynitrosamine. N-Nitroso-5-acetoxy-2-hydroxymorpholine was less reactive toward dG and gave the 1,N 2-etheno-dG adduct (44%). The products from both of the isomeric α-acetoxy nitrosamines were judged to arise from diazonium ions produced from unstable α-hydroxynitrosamine intermediates. The high yield of the rearrangement product 2-acetoxyacetaldehyde could explain the low carcinogenic potential of NHMOR if it is mainly α-hydroxylated at the 5 carbon. Hydroxylation of NHMOR at carbon 3 is expected to yield a carcinogenic outcome.