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  3. As people age, their cells become less efficient and less able replace damaged components. At the same time their tissues stiffe

As people age, their cells become less efficient and less able replace damaged components. At the same time their tissues stiffe

admin2011-01-02  51
问题    As people age, their cells become less efficient and less able 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, although few biologists recognized until recently that the same steps could take place in the body. Nonenzymatic glycosylation begins when an aldehyde group (CHO) of glucose and an amino group (HN2) of a protein are attracted to each other. The molecules combine, forming what is called a Schiff 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. These data suggest that nonenzymatic glycosylation of lens crystallins may contribute to cataract formation.
With which of the following statements concerning the stiffening of aging tissues would the author most likely agree?
选项 A、It paradoxically both helps and hinders the longevity of proteins in the human body.
B、It can be counteracted in part by increased ingestion of glucose-free foods.
C、It is exacerbated by increased enzymatic glycosylation.
D、It probably involves the nonenzymatic glycosylation of proteins.
答案D
解析 该题问：作者可能赞成下列哪一个与老年神经组织僵化有关的说法？D项意为“这可能涉及蛋白质无酶催化糖基化”。这在第一、二段可以说明。第一段讲述了人衰老，细胞功能减弱，其组织趋于僵硬，心、肺肌肉扩展差……变硬、韧带与腱紧缩等等。第二段提到“没有什么研究者会把这种不同的结果归咎于单个原因”。可是研究者发现长期已知的食物变色变硬的过程也许是促成跟老化相关的细胞和组织的损害。这个过程就是无酶催化糖基化。借此，“葡萄糖不用酶的协助就能依附于蛋白质”。A项意为“它自相矛盾地即帮助又制止人体内蛋白的帮助”；B项意为“它在增加摄取无葡萄糖食品作用下能部分地进行反作用”；C项意为“在增加了酶催化糖基化作用下，变硬会加剧”。
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