Free radicals as a cause of aging

1. Free radicals as a cause of aging

2. Many biologists believe that aging results from the gradual accumulation of damage to our body’s tissues. The most destructive damage probably occurs to DNA. 衰老是由逐渐积累的伤害作用于身体组织而引起的，变化首先发生在DNA方面

3. Alterations in DNA lead to the production of faulty genetic messages that promote gradual cellular deterioration. How does cellular damage occur? The answer may reside at the atomic level. • DNA改变—错误的遗传信息—细胞衰老

4. A.The form of free radicals Atoms are stabilized when their shells（壳） are filled with electrons. Electron shells consist of orbitals（轨道）, each of which can hold a maximum of two electrons.

5. Atom or molecules that have orbitals containing a single unpaired electron tend to be highly unstable----they are called free radicals. 自由基是指含有孤立的不成对价电子的极其不稳定的原子或分子。

6. Free radicals may be formed when a covalent bond（共价键） is broken such that each portion keeps one-half of the shared electrons, or they may be formed when an atom or molecule accepts a single electron transferred during an oxidation-reduction （氧化作用）reaction.

7. For example, water can be converted into free radicals when exposed to radiation from the sun: H2O OH- (hydroxyl radical)+ H+(hydrogen radical)

8. Free radicals are extremely reactive and capable of chemically altering many types of molecules, including proteins, nucleic acids, and lipids. The formation of hydroxyl radicals is probably a major reason that sunlight is so damaging to skin. 自由基具有反应活性，经化学变化可以转变为各类分子,如蛋白质、核酸、脂等 从羟基的形成可以看出太阳光对皮肤的伤害是很大的。

9. B. SOD(超氧化物歧化酶) In 1956, Denham Harman of the university of Nebraska proposed that aging results from tissue damage caused by free radicals. Because the subject of free radicals was not one with which biologists and physicians were familiar, the proposal failed to generate significant interest.

10. Then in 1969, JoeMcCord and Iriwin Feidovich of Duke University discovered an enzyme, superoxide dismutase(SOD超氧化物歧化酶), whose sole function was the destruction of the superoxide radical( O2- 超氧化自由基),a type of free radical formed when molecular oxygen picks up an extra electron.

11. SOD catalyzes（催化） the following reaction: O2- (superoxide radical) + O2- + 2H+ H2O2(hydrogen peroxide过氧化氢) + O2

12. Hydrogen peroxide（过氧化氢） is also a highly reactive oxidizing agent（氧化剂）, which is why it is often used a disinfectant（消毒剂） and bleaching agent（漂白剂）. If it is not rapidly destroyed, H2O2 can break down to form hydroxyl radicals （羟基） that attack the cell’s macromolecules. Hydrogen peroxideis（H2O2）normally destroyed in the cell by the enzymes catalase or glutathione peroxidase（谷胱甘肽过氧化物酶）.

13. C.superoxide(超氧化基) Subsequent research has revealed that superoxide radicals are formed within cells during normal oxidative metabolism and that a superoxide dismutase is present in the cells of diverse organisms, from bacteria to humans. 超氧化基形成于细胞内，当进行正常的新陈代谢氧化还原时，SOD可以存在于多种组织的细胞内（从细菌到人）。

14. In fact, animals possess three different versions of SOD: a cytosolic, mitochondrial, and extracellular version. • 动物有三种SOD存在形式。（细胞溶胶，线粒体，细胞外形式）

15. It is estimated that as much as 1-2 percent of the oxygen taken into human mitochondia can be converted to hydrogen peroxide（H2O2）rather than to water, the normal end product of respiration(呼吸作用）. The importance of SOD is most clearly revealed in studies of mutant bacteria and yeast that lack the enzyme; these cells are unable to grow in the presence of oxygen.

16. Similarly, mice that are lacking the mitochondrial version（线粒体型） of the enzyme(SOD2) are not able to survive more than a week or so after birth. Conversely, fruit flies genetically engineered（借助基因工程学改变） to produce large quantities of SOD can live more than 40 percent longer than untreated controls.

17. D. Antioxidants（抗氧化剂） A related area of research concerns the study of substances called antioxidants that are able to destroy free radicals. Common antioxidants found in the body include glutathion（谷胱甘肽）, vitamins E and C, and beta-carotene.

18. About the free radicals functions, SOD and vitamins will told more detailed by my parters heshu and liteng.

19. That’s all. Thank you!