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, 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 (NH2) 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.
17. With which of the following statements concerning the stiffening of aging tissues would the author most likely agree?
(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 is exacerbated by increased enzymatic glycosylation.（E）
(E) It probably involves the nonenzymatic glycosylation of proteins.
18. According to the passage, which of the following statements is true of the process that discolors and toughens foods?
(A) It takes place more slowly than glycosylation in the human body.
(B) It requires a higher ratio of glucose to protein than glycosylation requires in the human body.
(C) It does not require the aid of enzymes to attach glucose to protein.
(D) It proceeds more quickly when the food proteins have a molecular structure similar to that of crystallin proteins.（C）
(E) Its effectiveness depends heavily on the amount of environmental moisture.
19. According to the passage, which of the following is characteristic of enzymatic glycosylation of proteins?
(A) AGE’s are formed after a period of months or years.
(B) Proteins affected by the process are made unstable.
(C) Glucose attachment impairs and stiffens tissues.
(D) Glucose is attached to proteins for specific purposes.（D）
(E) Amino groups combine with aldehyde groups to form Schiff bases.
20. According to the passage, which of the following statements is true of Amadori products in proteins?
(A) They are more plentiful in a dehydrated environment.
(B) They are created through enzymatic glycosylation.
(C) They are composed entirely of glucose molecules.
(D) They are derived from Schiff bases.（D）
(E) They are derived from AGE’s.
21. Which of the following best describes the function of the third paragraph of the passage (lines 19-29)?
(A) It offers evidence that contradicts the findings described in the first two paragraphs.
(B) It presents a specific example of the process discussed in the first two paragraphs.
(C) It explains a problem that the researchers mentioned in the second paragraph have yet to solve.
(D) It evaluates the research discoveries described in the previous paragraph.（E）
(E) It begins a detailed description of the process introduced in the previous two paragraphs.
22. The passage suggests that which of the following would be LEAST important in determining whether nonenzymatic glycosylation is likely to have taken place in the proteins of a particular tissue?
(A) The likelihood that the tissue has been exposed to free glucose
(B) The color and spectrographic properties of structures within the tissue
(C) The amount of time that the proteins in the tissue have persisted in the body
(D) The number of amino groups within the proteins in the tissue（D）
(E) The degree of elasticity that the tissue exhibits
23. If the hypothesis stated in lines 56-58 is true, it can be inferred that the crystallin proteins in the lenses of people with cataracts
(A) have increased elasticity
(B) do not respond to enzymatic glycosylation
(C) are more susceptible to stiffening than are other proteins
(D) are at least several months old（D）
(E) respond more acutely than other proteins to changes in moisture levels