Nicotinamide Mononucleotide Determination of its Enzymatic Formation in vitro and its Physiological Rôle for the Biosynthesis of Nicotinamide‐Adenine Dinucleotide in Mice

A method is described to measure the activity of the NMN pyrophosphorylase (NMN: pyrophosphate phosphoribosyltransferase) in crude extracts of mouse tissues. The homogenate is incubated with 5‐phosphoribose‐1‐pyrophosphate, ATP, Mg2+ and [14C]nicotinamide in the presence of unlabelled NMN. Optimal conditions for the enzymatic assay were determined. The NMN pyrophosphorylase activity was measured in several organs of mice by this method. The observed activites were found to be considerably higher than reported in the literature. The formation of 14C‐labelled pyridine nucleotides in mouse liver after intraperitoneal injection of [14C]nicotinamide was studied. After the application of a high nicotinamide dose (1 mmole/kg) the biosynthesis of NAD was mainly via nicotinate. However, after the injection of a small nicotinamide dose within the physiological range the pathway via NMN is utilized to a large extent. Under these conditions a partial inhibition of the metabolic pathway of nicotinate to NAD has no influence on the NAD formation from nicotinamide. It is concluded that NMN is the physiological intermediate for NAD biosynthesis from nicotinamide.