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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
(5) become increasingly rigid, and the ligaments and tendons
tighten.
Few investigators would attribute such diverse effects
to a single cause. Nevertheless, researchers have discov-
ered that a process long known to discolor and toughen
(10)foods may also contribute to age- related impairment of
both cells and tissues. That process is nonenzymatic
glycosylation, whereby glucose becomes attached to pro-
teins without the aid of enzymes. When enzymes attach
glucose to proteins (enzymatic glycosylation), they do so
(15)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
(20)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
(25)attracted to each other. The molecules combine, forming
what is called a Schiff base within the protein. This com-
bination is unstable and quickly rearranges itself into a
stabler, but still reversible, substance known as an
Amadori product.
(30) 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 mol-
ecules to form irreversible structures named advanced
(35)glycosylation end products (AGE's). Most AGE's are
yellowish brown and fluorescent and have specific spec-
trographic 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
(40)no one has yet satisfactorily described the origin of all
such bridges between proteins, many investigators agree
that extensive cross-linking of proteins probably contrib-
utes to the stiffening and loss of elasticity characteristic
of aging tissues.
(45) In an attempt to link this process with the develop-
ment 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
(50) 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
(55)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) It probably involves the nonenzymatic glycosyla-
tion 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.
(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.
(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.
(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) It begins a detailed description of the process introduced in the previous two paragraphs.
