NADNADH

NADNADH refers to the redox couple formed by nicotinamide adenine dinucleotide in its oxidized form (NAD+) and its reduced form (NADH). It functions as a central electron carrier in cellular metabolism, cycling between NAD+ and NADH as electrons are transferred during oxidation-reduction reactions.

In energy metabolism, NAD+ accepts two electrons and a proton to become NADH, which then donates electrons to the mitochondrial electron transport chain, contributing to ATP production. NADH is produced during glycolysis, the pyruvate dehydrogenase reaction, and the citric acid cycle. Cytosolic NADH must be shuttled into mitochondria by systems such as the malate-aspartate shuttle or the glycerol-3-phosphate shuttle.

The cellular ratio of NAD+ to NADH reflects redox state and regulates metabolic flux. A high NADH level signifies a more reduced state and can inhibit several dehydrogenases, whereas a high NAD+ level supports catabolic pathways. NAD+ is synthesized through salvage pathways from nicotinamide or nicotinic acid (vitamin B3) and de novo from tryptophan in many organisms.

NADH should be distinguished from NADPH, a related cofactor used mainly in biosynthetic reactions and antioxidant defense. The NAD+/NADH pool is compartmentalized in the cytosol and mitochondria, and disturbances in its balance are investigated in relation to aging, metabolic disease, and mitochondrial function. Experimental approaches include supplementing NAD+ precursors such as nicotinamide riboside or NMN to modulate cellular NAD+ levels.