INTERACTION BETWEEN SUPEROXIDE-DISMUTASE AND DIPALMITOYLPHOSPHOTIDYLGLYCEROL BILAYERS - A FOURIER-TRANSFORM INFRARED (FT-IR) SPECTROSCOPIC STUDY

Purpose. Superoxide dismutase (SOD), an antioxidant enzyme, converts p eroxide radicals into hydrogen peroxide. Liposomes have been used as c arriers for SOD to enhance its antioxidant effect. Our previous DSC st udy has suggested that SOD binding to dipalmitoylphosphatidylglycerol (DPPG) may protect lipid membranes against oxygen-mediated injury. We now present FT-IR studies on the effect of DPPG binding on the tempera ture-induced SOD folding-unfolding process. Methods. The FT-IR spectra of SOD in D2O or DPPG membranes are measured as temperatures increase from 28 degrees to 121 degrees C at a rate of 0.5 degrees C/ min. Fro m the quantitative determination of the changes in the amide I band co mponents of the Fourier self-deconvoluted spectra, the DPPG-induced ch anges of SOD secondary structure could be detected as a function of te mperature. Results. We observe that the relative intensity of the SOD bands from 28 degrees C to 77 degrees C show graduate loss of beta-she et ''distorted'' structure, loss of turns, and existence of an interme diate state around 50 degrees C. Beginning at 80 degrees C, changes ar e obtained in three temperature regions: (i) 80 degrees C, (ii) 92 deg rees C, (iii) 109 degrees C. The result suggests that SOD folding/unfo lding transition involves mostly the relative changes within the regio ns of helix-like hydrogen bonding pattern, turn, twisted beta-bend and irregular structures. When SOD is bound to DPPG, the conformational c hanges shift to lower temperatures, indicating a reduction of SOD ther mal stability. In addition, the gel to liquid crystalline phase transi tion temperature of DPPG increases from 42 degrees C to 43.5 degrees C . Conclusions, These results suggest that the thermal stability of SOD is reduced by DPPG binding. However, DPPG bilayer is stabilized by th e presence of SOD.