首页
登录
职称英语
As people age, their cells become less efficient and less able to replace dam
As people age, their cells become less efficient and less able to replace dam
游客
2023-12-19
44
管理
问题
As people age, their cells become less efficient and less able to replace damaged components. At the same time their tissues stiffen. For example, the lungs and the heart muscle expand less successfully, the blood vessels become increasingly rigid, and the ligaments and tendons tighten.
Few investigators would attribute such diverse effects to a single cause. Nevertheless, researchers have discovered that a process long known to discolor and toughen foods may also contribute to age-related impairment of both cells and tissues. That process is nonenzymatic glycosylation, whereby glucose becomes attached to proteins without the aid of enzymes. When enzymes attach glucose to proteins (enzymatic glycosylation), they do so at a specific site on a specific protein molecule for a specific purpose. In contrast, the nonenzymatic process adds glucose haphazardly to any of several sites along any available peptide chain within a protein molecule.
This nonenzymatic glycosylation of certain proteins has been understood by food chemists for decades, al though few biologists recognized until recently that the same steps could take place in the body. Nonenzyrnatic glycosylation begins when an aldehyde group (CHO) of glucose and an amino group (NH2) of a protein are at tracted to each other. The molecules combine, forming what is called a Sehiff base within the protein. This combination is unstable and quickly rearranges itself into a stabler, but still reversible, substance known as an Amadori product.
If a given protein persists in the body for months or years, some of its Amadori products slowly dehydrate and rearrange themselves yet again, into new glucose-derived structures. These can combine with various kinds of molecules to form irreversible structures named advanced glycosylation end products (AGE’s). Most AGE’s are yellowish brown and fluorescent and have specific spectrographic properties. More important for the body, many are also able to cross-link adjacent proteins, particularly ones that give structure to tissues and organs. Although no one has yet satisfactorily described the origin of all such bridges between proteins, many investigators agree that extensive cross-linking of proteins probably contributes to the stiffening and loss of elasticity characteristic of aging tissues.
In an attempt to link this process with the development of cataracts (the browning and clouding of the lens of the eye as people age), researchers studied the effect of glucose on solutions of purified crystallin, the major protein in the lens of the eye. Glucose-free solutions remained clear, but solutions with glucose caused the proteins to form clusters, suggesting that the molecules had become cross-linked. The clusters diffracted light, making the solution opaque. The researchers also discovered that the pigmented cross-links in human cataracts have the brownish color and fluorescence characteristic of AGE’s. There data suggest that nonanzymatic glycosylation of lens crystallins may contribute to cataract formation. (454) [br] With which of the following statements concerning the stiffening of aging tissues would the author most likely agree?
选项
A、It is caused to a large degree by an increased rate of cell multiplication.
B、It paradoxically both helps and hinders the longevity of proteins in the human body.
C、It can be counteracted in part by increased ingestion of glucose-free foods.
D、It probably involves the nonenzymatic glycosylation of proteins.
答案
D
解析
推理判断题。本文的2、3、4段详细描述了在食物蛋白中存在的一种nonenzymatic glycosylation过程在人体内也同样存在,这一过程导致组织硬化、老化,因此D项为正确答案。
转载请注明原文地址:https://tihaiku.com/zcyy/3285566.html
相关试题推荐
CivilizationandHistoryMostofthepeoplewhoapp
CivilizationandHistoryMostofthepeoplewhoapp
CivilizationandHistoryMostofthepeoplewhoapp
1 Weoftenhearpeopleasksuchaquestion:Whydobadthingshappentogoodpe
1 Weoftenhearpeopleasksuchaquestion:Whydobadthingshappentogoodpe
[originaltext]Woman:Somepeopleareverycynicalaboutuh...televisionasam
Occasionally,wemaymeetsomeyoungpeoplewho,supposedlyhavingpassedtheco
1 Insomniaisunderappreciated.2 Manypeoplewouldratheradmittotheirfeti
1 Insomniaisunderappreciated.2 Manypeoplewouldratheradmittotheirfeti
Asignificantnumberofpeopleseektoadoptchildrenfromothercountries,a
随机试题
A.abuseB.acquiredC.affectD.appreciatedE.attractedF
A【B1】______ofthephysicalandchemicalnatureoflifemustbegin;noton
Ifyouwerelivinghere,____________________(你现在就会有和我们一样的感觉了).youwouldhaveth
电子商务信息系统的生命周期可以分为系统规划、系统分析、系统设计、系统实施、(
采用不通过血脑屏障显像剂的脑静态放射性核素显像,脑肿瘤的阳性率与下列的关系是A.
回访确认程序在投资冷静期内进行的无效,( ),投资者有权解除基金合同。A.回访
均可以用来治疗外感风寒表证、外感风热表证的药组是A.荆芥、防风 B.羌活、
油浸风冷变压器的控制箱必须满足当油温降低至()℃,且运行电流降到规定值时,风扇退
广电中心的防火要求:建筑面积()m2以上的录音室、演播室、候播厅、休息厅等应设置
消化性溃疡并发急性穿孔时,不可能出现A:板状腹 B:全腹压痛、反跳痛 C:肝
最新回复
(
0
)