Subcellular Compartmentation and Differential Catalytic Properties of the Three Human Nicotinamide Mononucleotide Adenylyltransferase Isoforms

Nicotinamide mononucleotide adenylyltransferase (NMNAT) is the central enzyme of the NAD biosynthetic pathway. Three human NMNAT isoforms have recently been identified, but isoform-specific functions are presently unknown, although a tissue-specific role has been suggested. Analyses of the subcellular localization confirmed NMNAT1 to be a nuclear protein, whereas NMNAT2 and -3 were localized to the Golgi complex and the mitochondria, respectively. This differential subcellular localization points to an organelle-specific, nonredundant function of each of the three proteins. Comparison of the kinetic properties showed that particularly NMNAT3 exhibits a high tolerance toward substrate modifications. Moreover, as opposed to preferred NAD+ synthesis by NMNAT1, the other two isoforms could also form NADH directly from the reduced nicotinamide mononucleotide, supporting a hitherto unknown pathway of NAD generation. A variety of physiological intermediates was tested and exerted only minor influence on the catalytic activities of the NMNATs. However, gallotannin was found to be a potent inhibitor, thereby compromising its use as a specific inhibitor of poly-ADP-ribose glycohydrolase. The presence of substrate-specific and independent nuclear, mitochondrial, and Golgi-specific NAD biosynthetic pathways is opposed to the assumption of a general cellular NAD pool. Their existence appears to be consistent with important compartment-specific functions rather than to reflect simple functional redundance.