Have you read biochem? When you do it you'll undersand. Actually G6PD is the major enzyme required for generation of NADPH ( in the step where g6pd converts glucose6P into 6phosphogluconate. NAPDH is produced as a byproduct. This NADPH has the capability to lend its hydrogen to glutathione thus reducing glutathione. The reduced glutathione causes scavenging of free radicals like H2O2 converting them into H2O. As a result glutathione is oxidized, now again we need to reduce this glutathione inorder for it to be effective again.. Which u already know takes place cz of G6PD.