Hydroxyl radical generating activity of hydrous but not calcined kaolin is prevented by surface modification with dipalmitoyl lecithin.

The catalytic activity of kaolin, an aluminum silicate, for generating hydroxyl radicals (.OH) from hydrogen peroxide (H2O2) was studied in a chemical system that measured .OH as evolution of methane (CH4) from dimethyl sulfoxide. In the presence of a reducing agent and 10 mM H2O2, hydrous and calcined kaolin generated mean +/- SE CH4 concentrations of 1634 +/- 328 and 1395 +/- 29 ppm, respectively. Surface modification with dipalmitoyl lecithin, the lipid of pulmonary surfactant, blocked generation of .OH in hydrous kaolin (38 +/- 38 ppm CH4) but not in calcined kaolin (875 +/- 262 ppm CH4). The catalytic activity of kaolin for producing .OH from H2O2 may be important in the pathogenesis of kaolin toxicity, and calcined kaolin may be more toxic than hydrous kaolin because the calcined form is resistant to surface modification by lipids of pulmonary surfactant.