Studies on endogenous formation of N-nitroso compounds in the guinea pig supplemented with proline or thioproline and sodium nitrate.

The endogenous formation of N-nitrosoproline (NPRO) and N-nitrosothioproline (NTPRO, N-nitrosothiazolidine-4-carboxylic acid) was studied by monitoring their excretion in the urine of guinea pigs given oral doses of 10 mg proline or thioproline after supplementation with 34 mg (0.4 mmol) sodium nitrate. In order to estimate the conversion of nitrate to nitrite, the animals were also supplemented with 3.5 mg (0.05 mmol) sodium nitrite instead of sodium nitrate. In animals fed commercial diets, the excretion of NPRO and NTPRO under supplementation with sodium nitrate was 2.0 micrograms and 28.7 micrograms/animal/day, respectively, whereas the excretion under supplementation with sodium nitrite was 0.7 micrograms and 13.3 micrograms/animal/day, respectively. The higher excretion of NTPRO than NPRO in each case shows that thioproline is more effective for nitrite trapping than proline. The animals supplemented with nitrate excreted more than twice the amounts of NPRO or NTPRO than those supplemented with nitrite. It is assumed, therefore, that more than 0.1 mmol nitrate is reduced to nitrite and takes part in the endogenous nitrosation of the guinea pig. When various concentrations of L-ascorbic acid (AsA), known to inhibit the formation of N-nitroso compounds, were also administered orally to animals immediately after supplementation with sodium nitrate, the NPRO excretion decreased with increasing AsA concentration. These data indicate that the guinea pig, which is unable to synthesize AsA as well as humans, may be an appropriate animal model for evaluation of the endogenous nitrosation ability of humans ingesting nitrate.