Nicotinamide methylation and its relation to NAD synthesis in rat liver tissue culture. Biochemical basis for the physiological activities of 1-methylnicotinamide.

The mode of [14C]nicotinamide conversion to NAD and 1-methylnicotinamide and the effects of exogenous 1-methylnicotinamide on this metabolic conversion were studied using rat liver slices incubated in a chemically defined culture medium. It was shown that at the physiological nicotinamide concentrations tested (11-500 microM), 1-methylnicotinamide is preferentially produced, rather than NAD. Upon increasing nicotinamide concentration to the levels that cause cytotoxicity (1-10 mM and higher), the rate of NAD synthesis dramatically increased and reached a level 6-fold higher than that of 1-methylnicotinamide. A dose-dependent inhibition (up to 60%) of NAD synthesis was seen by the exogenous addition of 1-methylnicotinamide; the degree of inhibition is affected also by the concentration of nicotinamide present as a precursor. A large depletion of intracellular ATP, associated with a marked accumulation of NAD, occurred in slices in response to the addition of high amounts of nicotinamide. However, the loss of ATP was overcome, when nicotinamide was given together with 1-methylnicotinamide. Finally, 1-methylnicotinamide per se was proven active in regulating cell growth by comparing the cytosolic activity of 1-methylnicotinamide oxidation of cultured RLC cells with that of rat liver. Thus, the previously observed growth stimulation of hepatic cells by 1-methylnicotinamide can reasonably been explained by its ATP-sparing effect due to the inhibition of NAD synthesis, a reaction which requires ATP.