Spectroscopic and calorimetric characterization of spermine oxidase and its association forms

Spermine oxidase (SMOX) is a flavin-containing enzyme that oxidizes spermine to produce spermidine, 3-aminopropanaldehyde and hydrogen peroxide. SMOX has been shown to play key roles in inflammation and carcinogenesis; indeed it is differentially expressed in several human cancer types. Our previous investigation has revealed that SMOX purified after heterologous expression in Escherichia coli actually consists of monomers, covalent homodimers, and other higher-order forms. All association forms oxidise spermine and, after treatment with DTT, revert to SMOX monomer.Here we report a detailed investigation on the thermal denaturation of SMOX and its association forms in native and reducing conditions. By combining spectroscopic methods (circular dichroism, fluorescence) and thermal methods (differential scanning calorimetry) we provide new insights into the structure, the transformation, and the stability of SMOX. While the crystal structure of this protein is not available yet, experimental results are interpreted also on the basis of a novel SMOX structural model, obtained in silico exploiting the recently solved acetylspermine oxidase crystal structure.We conclude that while at least one specific intermolecular disulfide bond links two SMOX molecule to form the homodimer, the thermal denaturation profiles can be justified by the presence of at least one intramolecular disulfide bond, which also plays a critical role in the stabilization of the overall three dimensional SMOX structure, and in particular of its FAD-containing active site.