The amount of encapsulated b-galactosidase as well as expressed b-galactosidase in bulk, inside the PEG-liposomes or bound onto Ni 2? -NTA liposomes were determined based on enzymatic activity of b-galactosidase using fluorescence spectroscopy (Amidi et al. 2010) . Briefly, in this assay, FDG as a substrate was cleaved by b-galactosidase into fluorescein, and two galactose molecules in a twostep reaction. By using high FDG concentrations relative to enzyme concentrations, the concentration of formed bgalactosidase could be directly deduced from the substrate conversion rate as measured by an increase in fluorescein fluorescence in time (excitation/emission wavelengths: 488/ 512 nm). A calibration curve of 0-500 lg/ml (corresponding to 0-200 IU/ml) of commercially available purified E. coli b-galactosidase was used in all experiments.