(A) Isotopically-labeled nicotinamide riboside (NR) or nicotinic acid (NAR) was synthesized to contain a C-13 on the pyridine carboxyl group and a deuterium on the ribose moiety (NAR labeling shown). (B) Fractional labeling of NAD in C2C12 whole cell lysate and isolated mitochondria following 4 hr of incubation with double-labeled NR with or without the NMNAT inhibitor, gallotannin (Gallo; 100 µM). (No label group, N = 1; NR 4 hr group, N = 4; NR + Gallo 4 hr group, N = 3). (C) Fractional labeling of NMN found in C2C12 whole cell lysate and isolated mitochondria following 4 hr of incubation with double-labeled NR with or without gallotannin (Gallo; 100 µM). (No label group, N = 1; NR 4 hr group, N = 4; NR + Gallo 4 hr group, N = 3). Data shown are means ± SEM and are representative of 2 independent experiments.