Fluorometric Determination of N-Nitroso-N-methylurea with Nicotinamide and Acetophenone

Abstract

It is well known that dietary secondary amides are readily nitrosatable intragastrically owing to the presence of acidic gastric juice and salivary nitrite ion; 1,2 also, many Nnitrosamides are direct-acting mutagens which show carcinogenicity to various laboratry animals. 3 Therefore, in addition to the roles of Helicobacter pylori infection, 4,5 Nnitrosamides formed in the stomach have been postulated to be potent etiological agents for gastric cancer in humans. 1,2,4,5 N-Nitroso-N-methylurea (NMU) is a strong carcinogenic Nnitrosamide; the formation of NMU in stomach has been noted because the dietary intake 6 and endogenous formation 7,8 of a precursor N-methylurea (MU) were suggested. Therefore, because the effects of foods on the N-nitrosation of MU under gastric conditions should be clarified, a suitable assay system is necessary. For the determination of N-nitrosamides including NMU, conventional photometric 9,10 and fluorometric 11 methods as well as various chromatographic methods [12][13][14][15] have been reported. In this paper, a sensitive and selective fluorometric method for the determination of NMU is described. The method is based on the N-methylation reaction of nicotinamide with NMU under a weak alkaline condition to form N 1 -methylnicotinamide (NMN) and a subsequent condensation reaction with acetophenone under an alkaline condition, followed by an acid treatment to give a fluorescent 2,7-naphthyridine derivative (Fig. 1). [16][17][18] The method was applied to a preliminaly study on the effect of foods on the N-nitrosation of MU under the conditions simulated to those of the stomach. 19 Fresh orange juice and milk were tested as model foods because they are reported to be protective factors concerning gastric cancer in humans. 1 Experimental ApparatusA Shimadzu RF-510 spectrofluorometer (Shimadzu, Kyoto, Japan) was used with a 0.4 × 1 cm quartz cell at room temperature. The spectral bandwidths were 5 nm (excitation) and 10 nm (emission). All fluorescence excitation and emission spectra were uncorrected. ChemicalsNicotinamide, N-nitroso-N-methylurethane, 1-iodobutane, 1,2-epoxyoctane (Tokyo Kasei, Tokyo, Japan), 2-methoxyethanol, acetophenone, acetonitrile, ethanol, dichloromethane, heptane, potassium hydroxide, sodium nitrite, potassium thiocyanate, sodium azide (Kanto Chemical, Tokyo, Japan), NMU, N-nitroso-N-ethylurea, N-nitroso-N-butylurea Shinjuku, Japan 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 µl 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...