Influence of cysteine and nitrate on the endogenous formation of N-nitrosamino acids.

The excretion of N-nitrosamino acids and nitrate in 24-h urine was studied over a continuous 45-day period for a single volunteer on a supplemented diet regime. Days 1-10 (normal diet), 11-20 (normal diet + 600 mg nitrate/day), 21-25 (normal diet), 26-30 (normal diet + 200 mg L-cysteine/day), 31-35 (normal diet), 36-40 (normal diet + 600 mg nitrate + 200 mg L-cysteine/day) and days 41-45 (normal diet). The presence of N-nitrosohydroxyproline (NHPRO) in human urine is reported for the first time as well as the influence of L-cysteine on the formation of N-nitrosothiazolidine-4-carboxylic acid (NTCA) and N-nitroso-2-methylthiazolidine-4-carboxylic acid (NMTCA) under a normal nitrate burden (75 mg/day) and a high nitrate burden (675 mg/day). After estimation of the amount of dietary nitrite consumed and the extent of nitrite formation by bacterial reduction of nitrate in the oral cavity, the total daily exposure to nitrite was calculated. From a total daily exposure of 7.2 mg nitrite/day from a typical European diet, 0.08% is required for the endogenous formation of N-nitrosamino acids found in urine. Under an increased nitrate burden (675 mg/day), the total exposure to nitrite was increased to 38 mg/day of which 0.14% was required for the endogenous formation on N-nitrosamino acids.