Variable SOD-like activity of liposome modified with Mn(II)-porphyrin derivative complex

A novel artificial superoxide dismutase (SOD), which can vary its activity in response to oxidative stress, was developed by using the liposome modified with a metal-functional ligand complex. A lipophilic chloro-(5,10,15,20- tetrakis[1-hexadecylpyridinium-4-yl]-21H,23H-porphyrin) (HPyP) was selected as the functional ligand to modify the liposome surface. The effects of the molar ratio of the HPyP and metal ion in the liposome membrane on two kinds of SOD-like functions, i.e. SOD activity and the antioxidative function against phospholipid composed of liposome was investigated by varying the membrane properties of liposomes. The modification of liposome with HPyP complex resulted in conflicting functions, such as an inhibitive and a promotional function during the induction of the antioxidation function against phospholipid, depending on the molar content of HPyP. The Mn(II)-HPyP complex can efficiently convert O₂^- to H₂O₂ and O ₂ at low HPyP content, while the efficiency of the SOD activity of Mn(II)-HPyP was significantly reduced due to the cluster formation of Mn(II)-HPyP complex in the liposome at a HPyP content exceeding 1 mol%. It was considered that the cluster formation of the Mn(II)-HPyP complex in the liposome may decrease the reactivity of the active center of the disproportional reaction of Mn(II)-HPyP since the complex was buried into the aggregate of Mn(II)-HPyP itself.