Unformatted text preview: lens of stiffen. For example, the lungs and the heart muscle expand less the eye. Glucose-free solutions remained clear, but solutions successfully, the blood vessels become increasingly rigid, and with glucose caused the proteins to form clusters, suggesting the ligaments and tendons tighten. that the molecules had become cross-linked. The clusters Few investigators would attribute such diverse effects to a diffracted light, making the solution opaque. The researchers single cause. Nevertheless, researchers have discovered that a also discovered that the pigmented cross-links in human process long known to discolor and toughen foods may also cataracts have the brownish color and fluorescence contribute to age- related impairment of both cells and tissues. characteristic of AGE's. These data suggest that That process is nonenzymatic glycosylation, whereby glucose nonenzymatic glycosylation of lens crystallins may becomes attached to proteins without the aid of enzymes. When contribute to cataract formation. 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
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
194 (454 words) F or the following question, consider each of the 5. Which of the following best describes the function choices separately and select all that apply of the third paragraph of the...
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- Fall '13