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Measurements were made of the effects of phenolic compounds, some of which are present in the human diet, on the nitrosation of proline by nitrite to give N-nitrosoproline (NPRO). In vitro, resorcinol, catechin, p-nitrosophenol and phenol were catalysts and chlorogenic acid an inhibitor; guaiacol showed a marginal catalytic effect. Both the catalytic and the inhibiting effects were dependent on pH and on the concentration of phenolic compounds; catalysis by resorcinol and catechin was increased at optimal ratios of [nitrite]: [phenolic compound]. Endogenous nitrosation was examined in vivo by co-administration of nitrite, proline and a phenolic compound to rats and by monitoring the amount of NPRO excreted in the urine. Under similar experimental conditions, the catalytic effects observed in vivo decreased in the same order as those observed in vitro: resorcinol greater than p-nitroso-phenol greater than catechin greater than phenol greater than or equal to guaiacol; chlorogenic acid acted as an inhibitor. Catalysis and inhibition of N-nitrosation in rats in vivo appears to occur via mechanisms similar to those in vitro, although the effects in vivo were smaller. The implications of our findings for the endogenous formation of N-nitroso compounds and for variations in exposure due to different dietary constituents in humans are discussed.

Various amounts of lyophilized beer were administered to rats dosed with proline and sodium nitrite. N-Nitrosoproline (NPRO) excreted in the 24-h urine was monitored as an index of endogenous nitrosation. In vitro formation of NPRO was determined after 15-min incubation of the same precursor solutions. Both in vivo and in vitro nitrosation of proline was inhibited in a dose-dependent fashion by lyophilized beers of different brands; the effects in vitro were most pronounced at pH below 4. The highest inhibitory effect was with beers with a high total polyphenolic content. Our results demonstrate that ingredients present in this widely consumed beverage inhibit endogenous nitrosation.