-!- The eukaryotic initiation factor eIF5A contains a hypusine residue that is essential for activity. -!- Catalyzes the first reaction of hypusine formation from one specific lysine residue of the eIF5A precursor. -!- The reaction occurs in four steps: NAD(+)-dependent dehydrogenation of spermidine (1a), formation of an enzyme-imine intermediate by transfer of the 4-aminobutylidene group from dehydrospermidine to the active site lysine residue (1b), transfer of the same 4-aminobutylidene group from the enzyme intermediate to the e1F5A precursor (1c), reduction of the e1F5A-imine intermediate to form a deoxyhypusine residue (1d). -!- Hence the overall reaction is transfer of a 4-aminobutyl group. -!- For the plant enzyme, homospermidine can substitute for spermidine and putrescine can substitute for the lysine residue of the eIF5A precursor. -!- Hypusine is formed from deoxyhypusine by the action of EC 1.14.99.29. -!- Formerly EC 1.1.1.249.