The present invention is a method for encapsulating active protein in a polymeric nanocarrier. The instant method employs homogenization at subzero temperatures so that enzyme activity is retained. Enzymes which can be encapsulated by the present method include, for example, antioxidant enzymes and xenobiotic detoxifying enzymes. Encapsulation of an enzyme protects it from protease degradation and increases therapeutic half-life. Advantageously, polymeric nanoparticles of the invention are permeable to enzyme substrates and therefore enzymes encapsulated by the instant method can exert their effect without release from the nanocarrier. Methods for decomposing a reactive oxygen species, protecting against vascular oxidative stress, and detoxifying a xenobiotic are also provided.