A novel NADPH:(bound) NADP⁺ reductase and NADH:(bound) NADP⁺ transhydrogenase function in bovine liver catalase

Many catalases have the shared property of containing bound NADPH and being susceptible to inactivation by their own substrate, H₂O₂. The presence of additional (unbound) NADPH effectively prevents bovine liver and human erythrocytic catalase from becoming compound II, the reversibly inactivated state of catalase, and NADP⁺ is known to be generated in the process. The function of the bound NADPH, which is tightly bound in bovine liver catalase, has been unknown. The present study with bovine liver catalase and [¹⁴C]NADPH and [¹⁴C]NADH revealed that unbound NADPH or NADH are substrates for an internal reductase and transhydrogenase reaction respectively; the unbound NADPH or NADH cause tightly bound NADP⁺ to become NADPH without becoming tightly bound themselves. This and other results provide insight into the function of tightly bound NADPH.