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.