2,5-Dimethylpyrazine and 2,3,5-trimethylpyrazine in natto are formed metabolically from threonine through aminoacetone by <i>Bacillus natto</i> during the fermentation

2,5-Dimethylpyrazine (2,5-DMP) and 2,3,5-trimethylpyrazine (T3MP) are flavor compounds in natto. Threonine seems to relate to the formation of these pyrazines by Bacillus subtilis var. natto (Bacillus natto), although the mechanism is unclear. When B. natto was cultured in minimal medium supplemented with L-threonine, L-glutamic acid, or L-serine, both 2,5-DMP and T3MP were formed only in the culture supplemented with L-threonine. Next, B. natto was cultured in the medium supplemented with 13C4, 15N-labeled L-threonine. 13C and 15N were incorporated into 2,5-DMP, T3MP, and aminoacetone during culture, suggesting that these compounds are formed from 13C4, 15N-labeled L-threonine through threonine metabolism by B. natto. When B. natto was cultured on steamed soybean, the amounts of threonine and aminoacetone increased, followed by the formation of 2,5-DMP and T3MP. These results show that 2,5-DMP and T3MP in natto are mainly formed through threonine metabolism by B. natto instead of Strecker degradation.