In order to study the mechanisms underlying the effects of TiO(2) nanoparticles on lactate dehydrogenase (LDH, EC1.1.1.27), Institute of Cancer Research region mice were injected with nanoparticulate anatase TiO(2) (5 nm) of various doses into the abdominal cavity daily for 14 days. We then examined LDH activity in vivo and in vitro and direct evident for interaction between nanoparticulate anatase TiO(2) and LDH using spectral methods. The results showed that nanoparticulate anatase TiO(2) could significantly activate LDH in vivo and in vitro; the kinetics constant (Km) and Vmax were 0.006 microM and 1,149 unit mg(-1) protein min(-1), respectively, at a low concentration of nanoparticulate anatase TiO(2), and 3.45 and 0.031 microM and 221 unit mg(-1) protein min(-1), respectively, at a high concentration of nanoparticulate anatase TiO(2). By fluorescence spectral assays, the nanoparticulate anatase TiO(2) was determined to be directly bound to LDH, and the binding constants of the binding site were 1.77 x 10(8) L mol(-1) and 2.15 x 10(7) L mol(-1), respectively, and the binding distance between nanoparticulate anatase TiO(2) and the Trp residue of LDH was 4.18 nm, and nanoparticulate anatase TiO(2) induced the protein unfolding. It was concluded that the binding of nanoparticulate anatase TiO(2) altered LDH structure and function.