Endogenous formation of N-nitrosoproline upon cigarette smoke inhalation

It was the goal of this study to assay the potential of inhaled cigarette smoke for endogenous N-nitrosation of amines in smokers by measuring urinary excretion of N-nitrosoproline (NPRO). Nonsmoking and smoking men were placed on a controlled diet which was low in proline and in ascorbic acid. On days 1-3, the volunteers received the controlled diet alone (Group I); on days 4-6, the diet was supplemented by a single daily dose of 300 mg proline (Group II); on days 7-9, the diet was supplemented by a single daily dose of 1 g ascorbic acid followed by 300 mg proline (Group III); and for the last three days, a single daily dose of 1 g ascorbic acid was given (Group IV). Collections of 24-h urine were made on days 3, 6, 9 and 12 of the study. The urine was analysed for NPRO, creatinine and cotinine. The mean 24-h NPRO excretion for 13 nonsmokers in Group I was 3.6 micrograms, whereas the NPRO excretion in 13 smokers was 5.9 micrograms/24 h, significantly higher than that of the nonsmokers (p less than 0.05). Urinary NPRO in 14 nonsmokers of Group II was significantly lower than that of the 14 smoking volunteers (p less than 0.05). In Group III smokers had reduced urinary levels of NPRO as a consequence of ascorbic acid intake. Differences in NPRO excretion by smokers and nonsmokers in Group IV were insignificant. These findings suggest that the documented endogenous N-nitrosation of proline which occurs as a result of cigarette smoke inhalation may also apply to other N-nitrosatable amines, including nicotine, and thus lead to in-vivo formation of carcinogenic N-nitrosamines. Four nonsmokers exposed to passive smoke for 80 min three times per day did not exhibit elevated NPRO levels in their 24-h urine.