Interaction between superoxide dismutase and dipalmitoylphosphotidylglycerol bilayers: a fourier transform infrared...

Superoxide dismutase (SOD), an antioxidant enzyme, converts peroxide radicals into hydrogen peroxide. Liposomes have been used as carriers for SOD to enhance its antioxidant effect. Our previous DSC study 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 temperature-induced SOD folding-unfolding process.

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. From the quantitative determination of the changes in the amide I band components of the Fourier self-deconvoluted spectra, the DPPG-induced changes of SOD secondary structure could be detected as a function of temperature.

We observe that the relative intensity of the SOD bands from 28 degrees C to 77 degrees C show graduate loss of beta-sheet "distorted" structure, loss of turns, and existence of an intermediate state around 50 degrees C. Beginning at 80 degrees C, changes are obtained in three temperature regions: (i) 80 degrees C, (ii) 92 degrees C, (iii) 109 degrees C. The result suggests that SOD folding/unfolding transition involves mostly the relative changes within the regions of helix-like hydrogen bonding pattern, turn, twisted beta-bend and irregular structures. When SOD is bound to DPPG, the conformational changes shift to lower temperatures, indicating a reduction of SOD thermal stability. In addition, the gel to liquid crystalline phase transition temperature of DPPG increases from 42 degrees C to 43.5 degrees C.

These results suggest that the thermal stability of SOD is reduced by DPPG binding. However, DPPG bilayer is stabilized by the presence of SOD.