Synthesis of N'-methylnicotinamide in X-irradiated Rats

Summary

The influence of 600 r total-body x-irradiation on the formation and excretion of N'-methylnicotinamide (N'-MeNA) has been investigated. A significant increase in the urinary excretion of N'-MeNA (67·5 per cent) was observed after irradiation. Studies of the in vitro N'-MeNA synthesis in liver homogenates revealed that synthetic activity decreased by 19·0 per cent in irradiated rats killed 1 hour later. Addition of ATP or glutathione to the reaction medium could not restore the diminished N'-MeNA synthesis, suggesting that the immediate fall in the synthesis of N'-MeNA was probably due to impairment of the enzymes involved in this methylation process. In vitro N'-MeNA synthesis at 20 and 24 hours after x-irradiation, however, was increased by 32·2 and 52·8 per cent respectively. This was corroborated by in vivo studies, which showed 40·5 per cent rise in N'-MeNA synthesis in x-irradiated rats. The evidence presented seems to indicate that in addition to the radiochemical degradation of methionine, the acceleration of transmethylation processes may be responsible for the lowering of methionine levels in x-irradiated animals.