Fig. 1.

Interplay between extracellular and intracellular pyridine metabolism. Schematic overview of the extracellular pyridine metabolome and the major reactions catalyzed by NAD-metabolizing ectoenzymes. Once imported into the cell, extracellular pyridine metabolites are used to generate intracellular NAD. Abbreviations of metabolites and enzymes are described in the text. Enzymes are sketched here, and in Figs. 2 and 3, based on their biological quaternary assembly, as determined by available 3D structures. Dashed arrows indicate metabolites’  fluxes or enzymes’ secretion