A water-soluble cationic 5, 10, 15, 20-tetrakis(2-dimethylsulfoniophenyl)-porphinatomanganese(III) ion (MnT2M(2)SuP) and a 5, 10, 15, 20-tetrakis(4-dimethylsulfoniophenyl)porphinatomanganese(III) ion (MnT4M(2)SuP) were synthesized as superoxide dismutase (SOD) mimics which were introduced into PEG-liposome composed of dimyristoylphosphatidylcholine (DMPC) and Pluronic F-68 to examine the effect of the liposome on the capacity for use as drug delivery system (DDS) to maintain and perpetuate blood circulation. Fluorescence spectra in pseudo blood circulation experiments indicated that MnT4M(2)SuP continued to be bundled in PEG-liposome, while fluorescence from cross-section of cell observed by confocal laser scanning microscope indicated that PEG-liposome was ingested into a cell. SOD activity was determined by stopped-flow analysis, which allowed the determination of k(cat) values for the reaction of the metalloporphyrins with superoxide anion radical (.O(2)(-)). Solution of PEG-liposome loaded with MnT2M(2)SuP or MnT4M(2)SuP were the most effective catalyst as a SOD mimic to decompose .O(2)(-) at second-order rate constants of 3.5-4.5 x 10(7) M(-1)s(-1).