Protein–nanoparticle hybridization can ideally lead to novel biological entities characterized by emerging properties, which can sensibly differ from those of the parent components. Herein, the effect of ionic strength on the biological functions of recombinant, His-tagged spermine oxidase (i.e., SMOX) was studied for the first time. Moreover, SMOX was integrated to colloidal surface active maghemite nanoparticles (SAMNs) by direct self-assembly, leading to a biologically active nano-enzyme (i.e., SAMN@SMOX). The hybrid was subjected to an in-depth chemical-physical characterization highlighting that protein structure was perfectly preserved. The catalytic activity of the nanostructured hybrid (SAMN@SMOX) was assessed by extracting kinetics parameters using spermine as substrate and compared to the soluble enzyme as a function of ionic strength. Results revealed that the catalytic function was dominated by electrostatic interactions and that they were drastically modified upon hybridization with colloidal ɣ-Fe2O3. Noteworthy, the affinity of SMOX toward spermine was significantly higher for the nanohybrid at low salinity. The present study supports the vision of using protein–nanoparticle conjugation as a mean for modulating biological functions.