Lo Y L, Rahman Y E
Department of Pharmaceutics, University of Minnesota, Minneapolis 55455, USA.
Pharm Res. 1996 Feb;13(2):265-71. doi: 10.1023/a:1016099232745.
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.
超氧化物歧化酶（SOD）是一种抗氧化酶，可将过氧自由基转化为过氧化氢。脂质体已被用作SOD的载体以增强其抗氧化作用。我们之前的差示扫描量热法（DSC）研究表明，SOD与二棕榈酰磷脂酰甘油（DPPG）的结合可能保护脂质膜免受氧介导的损伤。我们现在展示傅里叶变换红外光谱（FT-IR）研究，以探讨DPPG结合对温度诱导的SOD折叠-去折叠过程的影响。
以0.5℃/分钟的速率将温度从28℃升至121℃时，测量D2O或DPPG膜中SOD的FT-IR光谱。通过对傅里叶自卷积光谱酰胺I带成分变化的定量测定，可以检测到DPPG诱导的SOD二级结构变化与温度的函数关系。
我们观察到，从28℃到77℃，SOD条带的相对强度显示β-折叠“扭曲”结构逐渐丧失、转角丧失以及在50℃左右存在中间状态。从80℃开始，在三个温度区域出现变化：（i）80℃，（ii）92℃，（iii）109℃。结果表明，SOD的折叠/去折叠转变主要涉及螺旋状氢键模式、转角、扭曲的β-转角和不规则结构区域内的相对变化。当SOD与DPPG结合时，构象变化转移到较低温度，表明SOD热稳定性降低。此外，DPPG的凝胶-液晶相转变温度从42℃升高到43.5℃。
这些结果表明，DPPG结合会降低SOD的热稳定性。然而，SOD的存在使DPPG双层得以稳定。
