Binding of Human Serum Proteins to Titanium Dioxide Particles In Vitro

Abstract

Objectives: To determine the capacity of human serum proteins to bind to titanium dioxide (TiO₂) particles of different polymorphs and sizes. Methods: TiO₂ particles were mixed with diluted human serum, purified human serum albumin (HSA) or purified human serum gamma-globulin (HGG) solutions. After incubation at 37°C for 1 h, the particles were sedimented by centrifugation, and proteins in the supernatant, as well as those bound to the particles, were analyzed. Results: The total protein concentration in the supernatant was lowered by TiO₂, whereas the albumin/globulin ratio was elevated by the particles. Incubation with TiO₂ also lowered the immunoglobulin, pre-albumin, beta2-microglobulin, ceruloplasmin and retinol-binding protein levels, but not ferritin levels, in the supernatant. After sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), proteins in the supernatant, especially HGG, were observed to decrease, while those released from the particles (after adding 1% SDS and heating) increased, depending on the dose of TiO₂. Purified HGG and HSA were also bound to TiO₂, although the former appeared to have a higher affinity. All the proteins tested showed the highest binding potency to the amorphous particles (<50 nm) and the lowest to the rutile particles (<5,000 nm), while binding to anatase particles was intermediate. The affinity to the larger anatase was higher than that to smaller anatase particles in most cases. Conclusions: Human serum proteins, including the two major components, HSA and HGG, are bound by TiO₂ particles. The polymorph of the particles seems to be important for determining the binding capacity of the particles and it may affect distribution of the particles in the body.