Catalyzes the NAD-dependent oxidative cleavage of spermidine and the subsequent transfer of the butylamine moiety of spermidine to the epsilon-amino group of a specific lysine residue of the eIF-5A precursor protein to form the intermediate deoxyhypusine residue. Also able to produce homospermidine from putrescine (By similarity). [eIF5A protein]-L-lysine + spermidine = [eIF5A protein]-deoxyhypusine + propane-1,3-diamine Protein modification; eIF5A hypusination. Expressed in open and senescing flowers, but barely detectable in flower buds. Very low in fruits, until they began to soften. Not detectable in cotyledons after 5 days of germination, but reached a peak by day 15 when the chlorophyll levels began to decline. Up-regulated during senescence. Up-regulated by osmotic stress and chilling. Belongs to the deoxyhypusine synthase family.