ABSTRACT

Nicotinamide adenine dinucleotide (NAD+) is a key molecule for cellular energy metabolism. Its reduced form, NADH, serves as a carrier that stores chemical energy derived from oxidation of fuel molecules such as glucose and fatty acid. In aerobic organisms, the high-potential electron in NADH is transferred through the electron transport chain in mitochondria to oxygen. In this process of oxidative phosphorylation, ATP is generated to supply most of the energy requirement of cell functions, including the synthesis of NAD+ itself. The function of NAD+/NADH, as a high-potential electron transporter, intrinsically couples it to cellular energy status. Such a link puts NAD+ in a unique position suitable to reflect the overall environmental situations in terms of nutrient supply. The concentration of NAD+ could subserve as a sensor to

gauge whether conditions are favorable for optimal growth and propagation. However, when it is used as a coenzyme in the cycle of NAD+ NADH, the level of this pyridine nucleotide remains unchanged. Rather, the NAD+ level is affected by the rate of its synthesis and consumption as a substrate, which is greatly influenced by the extracellular stimuli, e.g., oxidative stress.