The redox coenzyme NAD+ is also a rate-limiting co-substrate for several enzymes that consume the molecule, thus rendering its continuous re-synthesis indispensable. NAD+ biosynthesis has emerged as a therapeutic target due to the relevance of NAD+ -consuming reactions in complex intracellular signaling networks whose alteration leads to many neurologic and metabolic disorders. Distinct metabolic routes, starting from various precursors, are known to support NAD+ biosynthesis with tissue/cell-specific efficiencies, probably reflecting differential expression of the corresponding rate-limiting enzymes, i.e. nicotinamide phosphoribosyltransferase, quinolinate phosphoribosyltransferase, nicotinate phosphoribosyltransferase and nicotinamide riboside kinase. Understanding the contribution of these enzymes to NAD+ levels depending on the tissue/cell type and metabolic status is necessary for the rational design of therapeutic strategies aimed at modulating NAD+ availability. Here we report a simple, fast and sensitive coupled fluorometric assay that enables simultaneous determination of the four activities in whole-cell extracts and biological fluids. Its application to extracts from various mouse tissues, human cell lines and plasma yielded for the first time an overall picture of the tissue/cell-specific distribution of the activities of the various enzymes. The screening enabled us to gather novel findings, including (a) the presence of quinolinate phosphoribosyltransferase and nicotinamide riboside kinase in all examined tissues/cell lines, indicating that quinolinate and nicotinamide riboside are relevant NAD+ precursors, and (b) the unexpected occurrence of nicotinate phosphoribosyltransferase in human plasma.
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