Figure 2.

The simplified pathways for nicotinamide, niacin, NAD⁺, and PARP-1 metabolism. Dietary nicotinamide, niacin, and tryptophan are precursors for the synthesis of NAD⁺, essential in ATP production and PARP-1 activation. Nicotinamide can be converted to niacin by bacterial nicotinamidase in the intestinal lumen. PARP-1 is activated by DNA strand breaks, cleaving NAD⁺ into nicotinamide and ADP-ribose. Poly(ADP-ribose) glycohydrolase (PARG) reactivates PARP-1 by removing poly(ADP-ribose)polymers, allowing for continuous NAD⁺ utilization. Figure is adapted from Jacobson et al., in 1995 [6] and Meyer-Ficca et al., in 2004 [12].