The biosynthesis and turnover of nicotinamide adenine dinucleotide in enucleated culture cells.

Abstract

AbstractNicotinic acid mononucleotide (NaMN) and nicotinamide mononucleotide (NMN) have been proposed as intermediates in the biosynthesis of nicotinamide adenine dinucleotide (NAD) from nicotinic acid and nicotinamide respectively. Although NaMN and NMN were not detected in acid extracts from whole D98/AH2 cells following pulses with 3H‐nicotinic acid and 3H‐nicotinamide, they were observed following pulses of enucleated cells. Several experiments indicate that enucleated cells are incapable of synthesizing NAD and that biosynthesis stops with mononucleotide formation. These results provide support for the idea, based upon fractionation studies, that NAD pyrophosphorylase (which catalyzes the reaction NMN or NaMN + ATP → NAD or desamido‐NAD) is localized exclusively in the nucleus.Acid extraction and chromatography were also used to compare the stability of 3H‐NAD in enucleated and whole cells. In the presence of the glutamine analog, azaserine, the intracellular concentration of 3H‐NAD decreased exponentially with a half‐life of 4 hours in whole cells. In enucleated cells the intracellular concentration of 3H‐NAD decreased with a half‐life greater than 14 hours. The increased stability of NAD in enucleated cells was confirmed by autoradiography. These results demonstrate the key role of the nucleus in both the biosynthesis and turnover of NAD.