A sensitive analytical procedure for the detection of N-nitrosamides via their alkylating activity

Fluorometric determination of N-nitroso-N-methylurea with nicotinamide and acetophenone

A fluorometric method for the determination of N-nitroso-N-methylurea (NMU) has been developed. It is based on the N-methylation reaction of nicotinamide with NMU and a subsequent condensation reaction with acetophenone, followed by an acid treatment to form a fluorescent 2,7-naphthyridine derivative. This method enabled the determination of NMU in the range 0.05 - 2 nmol/200 microl with a relative standard deviation of ca. 3%. It was applied to the determination of NMU formed from a precursor N-methylurea (MU) under simulated gastric conditions containing nitrite and thiocyanate ions at pH 3.0 in the presence of fresh orange juice and milk. NMU was extracted by an Extrelut 20 column and then determined. The mean recoveries of NMU added to the simulated gastric juice containing water, orange juice and milk were 86.5, 85.1 and 69.8%, respectively. The amounts of NMU formed from MU were found to decrease to below 25% in the presence of orange juice and milk.

N-Nitroso compounds in the diet

N-Nitroso compounds were known almost 40 years ago to be present in food treated with sodium nitrite, which made fish meal hepatotoxic to animals through formation of nitrosodimethylamine (NDMA). Since that time, N-nitroso compounds have been shown in animal experiments to be the most broadly acting and the most potent group of carcinogens. The key role of nitrite and nitrogen oxides in forming N-nitroso compounds by interaction with secondary and tertiary amino compounds has led to the examination worldwide of foods for the presence of N-nitroso compounds, which have been found almost exclusively in those foods containing nitrite or which have become exposed to nitrogen oxides. Among these are cured meats, especially bacon-and especially when cooked; concentrations of 100 micrograms kg(-1) have been found or, more usually, near 10 micrograms kg(-1). This would correspond to consumption of 1 microgram of NDMA in a 100-g portion. Much higher concentrations of NDMA (but lower ones of other nitrosamines) have been found in Japanese smoked and cured fish (more than 100 micrograms kg(-1)). Beer is one source of NDMA, in which as much as 70 micrograms l(-1) has been reported in some types of German beer, although usual levels are much lower (10 or 5 micrograms l(-1)); this could mean a considerable intake for a heavy beer drinker of several liters per day. Levels of nitrosamines have been declining during the past three decades, concurrent with a lowering of the nitrite used in food and greater control of exposure of malt to nitrogen oxides in beer making. There have been declines of N-nitroso compound concentrations in many foods during the past two decades. The small amounts of nitrosamines in food are nonetheless significant because of the possibility-even likelihood-that humans are more sensitive to these carcinogens than are laboratory rodents. Although it is probable that alkylnitrosamides (which induce brain tumors in rodents) are present in cured meats and other potentially nitrosated products in spite of much searching, there has been only limited indirect evidence of their presence.

Detection of methylating activity due to nitrosamide in some nitrosated Nigerian foodstuffs

Popular vegetables, condiments and some Nigerian staple foods were evaluated for their relative methylating potential due to nitrosamide formation following nitrosation under standardized conditions. Methylating activity of nitrosated foodstuffs, expressed as N-nitroso-N-methylurea equivalents, was determined by gas chromatography-thermal energy analysis. In positive samples (detection limit 10 microgram/kg) methylating activity detected was in the range of 50-1200 microgram/kg, the highest activity being found in Telfairia occidentalis (ugwu). This value constitutes the highest amount ever detected for a fresh vegetable. The data suggest that some commonly consumed local foodstuffs might contribute to overall human burden of environmental carcinogens in Nigeria.

Nitrosatable amines and nitrosamide formation in natural stimulants: Cola acuminata, C. nitida and Garcinia cola

Three varieties of kola nut, Cola acuminata, C. nitida and Garcinia cola, of Nigerian origin, were analysed for their content of primary and secondary amines, and assessed for their relative methylating potential due to nitrosamide formation. Primary and secondary amines were determined as benzene sulfonamides by gas chromatography/thermal energy analysis (GC/TEA). Dimethylamine, methylamine, ethylamine and isopentylamine were detected in all kola nut varieties, while pyrrolidine, piperidine and isobutylamine were detected in one or more varieties. Estimated average total daily intake of aliphatic amines by a typical kola nut chewer varied from 260 to 1040 micrograms/day for secondary amines and from 2430 to 9710 micrograms/day for primary amines. Methylating activity of the nitrosated kola nuts, expressed as N-nitroso-N-methylurea equivalents, was also determined by GC/TEA. Methylating activity was significantly higher in kola nuts (170-490 micrograms/kg) than has ever been reported for a fresh plant product. These data suggest that the possible role of kola nut chewing in human cancer aetiology should be explored in countries where kola nuts are widely consumed as stimulants.

High-performance liquid chromatographic determination of N-nitroso-N-alkylureas by pre-column fluorescence derivatization and application to blood analysis

A method for the derivatization and separation of N-nitroso-N-alkylureas [alkyl = methyl (NMU), ethyl (NEU), and n-butyl (NBU)] has been developed. Fluorescent derivatives were formed with sodium sulphide, taurine and o-phthalaldehyde and separated by reversed-phase high-performance liquid chromatography. The limits of detection of standard NMU, NEU and NBU were 0.25, 0.8 and 1.5 pmol/200 microliters, respectively. The method was applied to the determination of NMU in blood after extraction with acetonitrile in the presence of calcium chloride. NBU was used as the internal standard. The recovery of NMU from blood was ca. 95%, and the limit of detection was 10 pmol/400 microliters blood. NMU levels in rabbit blood following a single oral administration were also measured.

Review of the occurrence and formation of non-volatile N-nitroso compounds in foods

A review of the literature published prior to July 1991 covers the occurrence and formation of non-volatile N-nitrosamines occurring in foods and beverages. The presence of identified volatile and non-volatile N-nitrosamines accounts for less than 10% of the total apparent N-nitrosamine concentration. N-Nitrosoproline and N-nitrosothiazolidine-4-carboxylic acid are the most commonly identified non-volatile N-nitrosamines in the diet. Non-volatile N-nitrosamines account for 12 of the 21 currently identified N-nitroso compounds in foods and beverages.

Trace analysis of nitrosated foodstuffs for nitrosamides

An analytical procedure previously developed for the trace determination of nitrosamides was applied to a screening of nitrosated foodstuffs for nitrosoureas. Different types of foodstuffs were nitrosated both under chemical conditions using a high nitrite concentration, and under simulated gastric conditions. Methylating activity corresponding to N-nitroso-N-methylurea (MNU) was detected in most samples. Under chemical conditions, the yields spanned several orders of magnitude with processed fish and meat products being at the top, and plant products at the bottom of the scale. After nitrosation under simulated gastric conditions, the range of MNU activity was significantly smaller. No correlation exists between the yields determined under chemical and simulated gastric conditions.

N-nitroso compounds and their precursors in Brassica oleracea

In the present study, Brassica a dried green vegetable from Kashmir, which is a major constituent of the local diet, was analysed for nitrosatable aliphatic amines, N-nitrosamines prior to and after nitrosation) and alkylating activity due to N-nitrosamides following nitrosation. The cooked vegetable contained 11 micrograms/kg nitrosodimethylamine and 21 micrograms/kg nitrosopyrrolidine. Nitrosation under chemical conditions yielded 1200 micrograms/kg N-methylnitrosourea.