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Free radicals
as a cause of
aging
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
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
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.
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。
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.
For example, water
can be converted into
free
radicals
when
exposed to radiation
from the sun:
H2O
radical)+
(hydroxyl
OH
+(hydrogen
radical)
H
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。
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.
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.
SOD catalyzes （ 催 化 ）
the following reaction:
O2 (superoxide radical) +
+
O2 + 2H
H2O2(hydrogen peroxide 过 氧 化
氢) + O2
-
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）.
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）。
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）
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.
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.
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 betacarotene.
About the free radicals
functions, SOD and
vitamins will told more
detailed by my parters
heshu and liteng.
That’s all.
Thank you!