Cellular Compartmentation and the Redox/Non-Redox Functions of NAD⁺.

<b>Significance:</b> Nicotinamide Adenine Dinucleotide (NAD⁺) spans diverse roles in biology, serving as both an important redox co-factor in metabolism and a substrate for signaling enzymes that regulate protein post-translational modifications (PTMs). <b>Critical Issues:</b> Although the interactions between these different roles of NAD⁺ (and its reduced form NADH) have been considered, little attention has been paid to the role of compartmentation in these processes. Specifically, NAD⁺'s role in metabolism is compartment specific (e.g., mitochondrial vs. cytosolic), affording a very different redox landscape for PTM-modulating enzymes such as sirtuins and poly (ADP-ribose) polymerases in different cell compartments. In addition, the orders of magnitude differences in expression levels between NAD⁺-dependent enzymes is often not considered when assuming the effects of bulk changes in NAD⁺ levels on their relative activities. <b>Recent Advances:</b> In this review, we discuss the metabolic, non-metabolic, redox, and enzyme substrate roles of cellular NAD⁺, and the recent discoveries regarding the interplay between these roles in different cell compartments. <b>Future Directions:</b> Therapeutic implications for the compartmentation and manipulation of NAD⁺ biology are discussed.