Effect of vitamins C and E on endogenous synthesis of N-nitrosamino acids in humans: precursor-product studies with [15N]nitrate.

The endogenous formation of nitrosoproline (NPRO) following administration of nitrate and proline is reported in ten healthy young adults. There was a relatively constant basal excretion of NPRO, 26 +/- 10 (SD) nmol/day, in excess of amounts found in the diet. This basal synthesis of NPRO was not reduced by ascorbic acid (2 g/day) or alpha-tocopherol (400 mg/day). A significant rise in the excretion of NPRO was observed following the administration of nitrate and proline, ranging from 29 to 318 nmol/24 h with a mean of 100 nmol/24 h. [15N]Nitrate was used as a tracer to study the observed excess excretion of NPRO in urine. The data revealed that urinary NPRO excretion as a result of endogenous synthesis is not totally derived from ingested nitrate as its precursor. The ingestion of ascorbic acid and alpha-tocopherol inhibited the incorporation of [15N]nitrate into NPRO by 81 and 59%, respectively. An additional nitrosamino acid, N-nitrosothiazolidine-4-carboxylic acid, was present in the urine. It was found that N-nitrosothiazolidine-4-carboxylic acid increased 6-fold upon ingestion of nitrate. Ascorbic acid and alpha-tocopherol blocked this nitrate induced synthesis.