**4.2. The importance of pentose phosphate pathway for erythrocytes**

G6PD is the key enzyme in the oxidative pentose phosphate pathway. The first step of the pentose phosphate pathway is catalyzed by G6PD. In this step, NADP+ is reduced to NADPH, and ribulose-5-phosphate, a precursor of DNA, RNA, and ATP, emerge from G6P (Turner, 2000). The most important reducing agent in the cytoplasm of cells is NADPH (Koehler & Van Noorden, 2003). The second enzymatic step in this pathway is NADPH production as a consequence of reactions that reduce oxidized glutathione (GSSG) to reduced glutathione (GSH). The only defense against oxidant stress in the red blood cell (RBC) is GSH production (Friedman, 1979, Group, 1989, Peters & Van Noorden, 2009). In unstressed, normal erythrocytes, the G6PD activity is only about 2% of total capacity (Group, 1989, Peters & Van Noorden, 2009). The pentose phosphate pathway's main function is the generation of reducing capacity through the production of NADPH and ultimately, GSH. This is essential for cell survival and is available in the erythrocyte for generating reducing capacity (Greene, 1993).
