Assays for Determination of Cellular and Mitochondrial NAD⁺ and NADH Content

NAD⁺ is a redox cofactor essential to the proper functioning of a variety of important metabolic pathways, including key steps in mitochondrial energy metabolism. In addition, it serves as a signaling substrate for enzymes such as sirtuins and the poly-ADP ribosyl-polymerase family of enzymes. Sirtuins, which are NAD⁺-dependent protein deacylases, harness changes in cellular NAD⁺ concentrations to produce changes in protein acylation status, thereby affecting downstream functions including energy metabolism, stress resistance, and cell survival. Thus, the availability of NAD⁺ in cells, or in specific organelles such as the mitochondrion, regulates downstream signaling and key biological processes. This concept has driven a need for researchers to easily and precisely measure NAD⁺ concentrations in biological samples. We herein describe several protocols for the measurement of NAD⁺ and NADH concentrations in tissues, cells, or subcellular compartments such as mitochondria. These protocols include a cycling assay that can quickly measure NAD⁺ or NADH levels using a plate reader equipped with fluorescence measurement capabilities. This plate assay relies only upon commercially available materials in addition to the biological samples of interest. In addition, we describe a protocol employing stable isotope-labeled NAD⁺ as an internal standard to determine biological NAD⁺ content by isotope-dilution methods. This method requires mass spectrometry to ratio endogenous NAD⁺ with exogenous isotope-labeled NAD⁺ to obtain quantification using HPLC and mass spectrometry.