Cytosolic deoxyhypusine synthase (DHPS) tetramer catalyzes the reaction of of EIF5A protein, spermidine (SPM), and NAD+ to convert lysine 50 of EIF5A to deoxyhypusine (Dhp), generating 1,3 diaminopropane, NADH and H+ in the process (Clement et al. 2003; Joe et al. 1995; Park 2006; Wolff et al. 1997). Although the reaction is reversible, the reverse reaction is probably minimized under physiological conditions by the rapid, irreversible conversion of EIF5A Dhp residues to hypusine.