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In the present study,poly(amidoamine) (PAMAM) encapsulated platinum nanoparticles were synthesized and used as catalase mimics.Acetylated generation 9 (G9) PAMAM dendrimer (Ac-G9) with a molecular size around 10 nm were used as the platinum nanoparticle synthesis template.The feeding molar ratio of Pt4+ and Ac-G9 is 2048 and the synthesized platinum nanoparticles have an average size of 3 nm.Ac-G9 (Pt2048) has a similar molecular size (~11 nm)and globular shape with catalase (Figure 1).The catalytic activity of Ac-G9 (Pt2048) on the degradation of H2O2 is approaching that of catalase at 37 ℃.Interestingly,Ac-G9 (Pt2048) showed differential response to the changes of pH and temperature,which can be explained by different catalytic mechanisms of Ac-G9 (Pt2048) and catalase.Furthermore,Ac-G9 (Pt2048) can be efficiently uptake by HeLa cells and shows excellent biocompatibility on different cell lines.It can down-regulate the H2O2-induced intracellular reactive oxygen species (ROS) in these cells.These results suggest that dendrimers are promising mimics of proteins with different sizes and Ac-G9 (Pt2048) can be used as an alternative candidate of catalase to decrease the oxidation stress in ceils.