Influence of ascorbic acid dose on N-nitrosoproline formation in humans.

A relationship between ascorbic acid intake and N-nitrosoproline (NPRO) excretion in humans on a controlled diet was established. Seven healthy males were placed on a low nitrate, low ascorbic acid diet for 12 consecutive days. On days 3-12, a 5.24 mmol oral dose of sodium nitrate was administered in mid-afternoon, at least 2 h after the subject's last meal. On days 4-12, a 4.35 mmol oral dose of L-proline was administered 30 min after the nitrate dose. Ascorbic acid was given in amounts which increased daily from day 5 to day 10 (0.01-5.68 mmol; 1.76-1000 mg) with the proline. Total 24 h urines were assayed for nitrate, NPRO and total ascorbic acid. Nitrate balance was monitored using [15N]nitrate. Average endogenous nitrate synthesis was 1.28 +/- 0.43 mmol/day/person. NPRO excretion was reduced by 6 nmol/day when 0.05 mmol of ascorbic acid was administered. However, as much as 5.68 mmol ascorbic acid did not return NPRO excretion to levels observed before the nitrate and proline were administered. More than 10 times the ascorbic acid required to completely inhibit NPRO formation in vitro did not return NPRO excretion to baseline levels. These data indicate that endogenous nitrosation may be more facile than predicted by the in vitro chemistry.

Relationship between ascorbic acid dose and N-nitrosoproline excretion in humans on controlled diets.

A logarithmic dose-response relationship between ascorbic acid dose and N-nitrosoproline (NPRO) excretion in humans on a controlled diet was established. Seven healthy males were placed on a low-nitrate, low-ascorbic acid diet for 12 consecutive days and given nitrate on days 3-12 and L-proline on days 4-12, after the nitrate dose. Ascorbic acid was given in increasing amounts with the proline on days 5-10. Urine was analysed quantitatively for nitrate, NPRO and ascorbic acid. Ascorbic acid doses as low as 0.05 mmol reduced NPRO excretion by an average of 6 nmol/day; however, as much as 5.68 mmol ascorbic acid did not return NPRO excretion to levels observed before nitrate and proline were administered. Complete inhibition of endogenous NPRO formation from exogenous precursors requires more than the 2:1 molar ratio of ascorbic acid to nitrite that has been demonstrated in vitro. These data may be useful in interpreting epidemiological studies of nitrate exposure and in making dietary recommendations.

Basal N-nitrosoproline formation and excretion in the ferret.

Quantitative balance studies indicated that ferrets (Mustela putorius furo) on purified diets excreted approximately four times more N-nitrosoproline than they ingested. This basal N-nitrosoproline excretion was not dependent on dietary nitrate intake, as high levels of dietary [15N]nitrate did not result in the incorporation of 15N into the excreted N-nitrosoproline. This indicates that there are at least two sources of endogenous N-nitrosoproline formation, one linked to dietary nitrate and one independent of dietary nitrate. The latter may result from an intermediate in the endogenous synthesis of nitrate.