Methylation of nicotinamide with soluble enzyme system from rat liver.

Handler and Dann (1) suggested that nicotinamide is methylated in viva by methionine and excreted in the urine as N’-methylnicotinamide (NMeN). Perlzweig, Bernheim, and Bernheim (2) demonstrated that NMeN is formed in vitro by rat liver slices. The amounts formed were quite small and the maintenance of aerobic conditions and of cellular integrity was essential; furthermore, the addition of methionine in vitro did not lead uniformly to an increased formation of NMeN. These findings have since been confirmed and extended by Ellinger (3), who, like the earlier investigators, failed to obtain methylation of nicotinamide in a cell-free extract of rat liver. The earliest observations pointing to the transfer of the methyl group of methionine to a suitable methyl acceptor in a cell-free system are those of Borsook and Dubnoff (4). These authors established that guinea pig liver homogenates supplemented with adenylic acid, an oxidizable substrate such as ar-ketoglutaric acid, and oxygen formed creatine from guanidoacetic acid and L-methionine. In recent years it has become increasingly clear that the requirement for oxygen in synthetic reactions with tissue slices or homogenates is often related to the aerobic generation of energy-rich phosphate bonds.’ In some cases it has actually been possible to eliminate the requirement for aerobiosis by demonstrating that adenosinetriphosphate (ATP) is utilized anaerobically as a source of energy for endergonic reactions (5-9). Since methionine is the methyl donor for the methylation of both nicotinamide and guanidoacetic acid, it was considered of interest to investigate the