Effect of nucleotides on the oligomeric state of human lactoferrin

Lactoferrin (LF) is a multifunctional acute-phase protein involved in nonspecific defense against bacteria, viruses, and cancer diseases and is present in human barrier fluids, blood, and milk. Small-angle X-ray scattering (SAXS) and light scattering (LS) demonstrated for the first time that LF occurs in the form of oligomers, with a high monomer unit number in the solution. The degree of LF oligomerization depends on the LF concentration and the storage period of non-frozen neutral LF solutions. The average inertial radius of scattering particles (R _g) reaches 100–450 Å at LF concentrations comparable with those in human milk, while R _g of LF monomers is 26.7 Å. LF forms complexes with various nucleotides and hydrolyzes them. The addition of ATP or AMP to LF solutions accelerates LF oligomerization and increases R _g to 600–700 Å, regardless of the initial degree of LF oligomerization. According to the different models (sphere, plate, and cylinder) of LF aggregates, its complexes with such R _g presumably contain several tens to thousands of LF monomers. The possible role of oligomeric complexes in multiple biological functions of LF is discussed.