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This study investigated the nitrosating activities of intact cells and cell fractions of Bacillus cereus isolated from fermented palm wine [Elaeis guineensis] during incubation with dimethylamine and nitrite or nitrate. The pH values of the incubation mixtures were 7.2 ± 0.07 and 7.1 ± 0.07 in the presence of nitrite and nitrate, respectively. The N-Nitrosodimethylamine [NDMA] level in cell debris was significantly higher compared with cell extract, cell suspension and sterile controls. The time-course of NDMA formation in cell debris showed a significant [p<0.05] early decline followed by an increase. The kinetic data obtained from Lineweaver-Burk plots of NDMA formation showed that Michaelis -Menten constant [Km] value was 37.5% lower, while initial velocity [Vmax] was 20.0% higher in cell debris relative to cell extract, when a fixed DMA concentration was used against varying nitrite concentrations. However, at a fixed nitrite concentration against varying DMA concentration, Km value was 44.4% lower, while Vmax was 166.7% higher in cell debris compared with cell extract. Investigation of nitrosation mechanism showed that Vmax values in the cell debris were 41.7, 50.0 and 55.6 μmol NDMA/mg protein, while the values were 34.5, 37.0 and 43.5 μmol NDMA/mg protein in cell extract for the respective three nitrite concentrations.
This study has shown that intact Bacillus cereus cells catalyzed nitrosation of dimethylamine at near neutral pH, and the nitrosation, which was higher in cell debris than cell extract of the bacterium, followed a sequential mechanism of enzyme catalysis.
