Studies on the Biosynthesis of NAD from Nicotinamide and on the Intracellular Pyridine Nucleotide Cycle in Isolated Perfused Rat Liver

Abstract

Isolated perfused rat liver incorporates nicotinamide at a concentration of 10 μM into NAD via nicotinamide mononucleotide. A deamidation of nicotinamide could not be demonstrated. Evidence is presented which indicates that the exchange reaction catalyzed by NAD glycohydrolase is not involved in the uptake of nicotinamide into NAD. At the concentrations used, the rate of the incorporation of nicotinamide into NAD is about 40% of the incorporation rate observed with nicotinic acid. Results obtained after perfusion with [ 14 C]nicotinic acid support the concept of an intracellular pyridine nucleotide cycle. However, in contrast to the general opinion, our results demonstrate that in rat liver nicotinamide generated by NAD breakdown is not deamidated but reutilized via nicotinamide mononucleotide. After perfusion with 0.1 mM [ 14 C]nicotinamide for 2.5 h, less than 2% of the total radioactivity is excreted into the bile. The same value is obtained after perfusion with 50 μM [ 14 C]nicotinic acid. From these results it is concluded that the excretion of nicotinamide by the liver and the subsequent deamidation within the intestinal tract which have been considered important in the utilization of nicotinamide by the liver, are without biological relevance at normal nicotinamide concentrations.