This student paper was written as an assignment in the graduate course Free Radicals in Biology and Medicine

Nicotinamide adenine dinuclotide phosphate reduced form (NADPH), an electron carrier, is a kind of antioxidants against the oxidative stress in the mitochondria. The major resources of NADPH are the pentose phosphate pathway and photosynthesis. This paper will focus on the structure, formation, reaction, detection and stability of NADPH. Jingru Liu NADPH 3 Introduction: Recently, reactive oxygen species (ROS), which are generated by oxidative stress, are considered as cytotoxic products of cellular metabolism and play an important role in the growth, division, transformation, and apoptosis in mammalian cells. ROS has been involved in many human diseases, such as aging, diabetes, stroke and many types of cancers. The damages induced by ROS attack take place in mitochondria that has been demonstrated as one of the major sources of ROS. ROS can directly damage mitochondrial enzymes [1], and cause mitochondrial DNA mutation [2]. In the process of these damages in mitochondria, superoxide anions (O2•) are produced by the leakage of electrons, and then rapidly reduced to H2O2 by manganese superoxide dismutase (SOD). H2O2 can be converted to H2O by the function of catalase. However, catalase is absent in the mitochondria of most animal cells, mitochondrial glutathione peroxidase (GPx) plays a key role in metabolizing H2O2. Reduced glutathione (GSH) known as an efficient radical scavenger is very important for the activity of mitochondrial GPx. GSH is known to be synthesized in the cytosol and transported into the mitochondria [3-4]. GSH converts to oxidized glutathione disulfide (GSSG) in the mitochondria which cannot be exported into the cytosol for reconversion into GSH. Therefore, mitochondrial NADPH becomes a necessary reducing equivalent for the regeneration of GSH from GSSG by the activity of mitochondrial glutathione reductase (GR). Because of the importance of NADPH in reductive system, it becomes one of the best antioxidant available against the oxidative stress in the mitochondria. . Structure of NADPH NADPH is a water-soluble two-electron carrier. NADP+ is the oxidized form which lacks electrons. Nicotinamide nucleotide-linke dehydrogenase can catalyze the reversible reaction that NADP+ accept electron and convert to NADPH (reaction 1). Compared to NADH, which Jingru Liu NADPH 4 donates electrons to the electron transport system for energy generation, NADPH donates electrons to biosynthetic reactions [5]. NADPH+ NADH+ ↔ NADPH+ NAD+ (reaction 1) In cells, NADPH is produced by the pentose phosphate pathway. In plants, photosynthesis is another source of NADPH. Figure1: The structure of NADPH and NADP+ [5] NADP+ can accept one electron and convert to NADPH. The reaction mainly takes place in mitochondria. Two major pathway of NADPH generation Pentose Phosphate Pathway In biosynthetic reactions, the pentose phosphate pathway is an importance of generating NADPH. It operates exclusively in the cytosol. The pentose phosphate pathway includes oxidative phase which produces NADPH, and non-oxidative phase which produce pentoses and miscellaneous other sugar phosphates [6]. In oxidative phase, there exist four reactions. Glucose6-phosphate dehydrogenase is the key enzyme that is responsible for the generation of NADPH [7]. Evidence shows that it reduces cellular oxidative stress by increasing the GSH concentration [8].