Science for University Students. Part II. Translations. Сологуб Л.И. - 53 стр.

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human health. In the meantime, real hazards are not receiving adequate
attention.
The current procedures for gauging carcinogenicity are coming under
increasing scrutiny and criticism. A leader in the examination is Bruce Ames,
who with others has amassed an impressive body of evidence and arguments.
Ames and Gold summarized some of their recent data and conclusions in
Science (31 August, p. 970). Three articles in press in the Proceedings of the
National Academy of Sciences provide an elaboration of the information with
extensive bibliographies. The articles also provide data about other pathologic
effects of natural chemicals.
A limited number of chemicals tested, both natural and synthetic, react with
DNA to cause mutations. Most chemicals are not mutagens, but when the
maximum tolerated dose (MTD) is administered daily to rodents over a lifetime,
about half of the chemicals give rise to excess cancer, usually late in the normal
life span of the animals. Experiments in which synthetic industrial chemicals
were administered in the MTD to both rats and mice resulted in 212 of 350
chemicals being labeled as carcinogens. Similar experiments with chemicals
naturally present in food resulted in 27 of 52 tested being designated as
carcinogens. These 27 rodent carcinogens have been found in 57 different foods
including apples, bananas, carrots, celery, coffee, lettuce, orange juice, peas,
potatoes, and tomatoes. They are commonly present in quantities thousands of
times as great as are the synthetic pesticides.
The plant chemicals that have been rested represent only a tiny fraction of
the natural pesticides. As a defense against predators and parasites; plants have
evolved a large number of chemicals that have pathologic effects on their
attackers and consumers. Ames and Gold estimate that plant foods contain 5,000
to 10,000 natural pesticides and break-down products. In cabbage alone some 49
natural pesticides have been found. The typical plant contains a total of a percent
or more of such substances. Compared to the amount of synthetic pesticides we
consume, we eat about 10,000 times more of the plant pesticides.
It has long been known that virtually all chemicals are toxic if ingested in
sufficiently high doses. Common table salt can cause stomach cancer. Ames and
others have pointed out that high levels of chemicals cause large-scale cell death
and replacement by division. Dividing cells are much more subject to mutations
than quiescent cells. Much of the activity of cells involves oxidation, including
formation of highly reactive free radicals that can react with and damage DNA.
Repair mechanisms exist, but they are not perfect. Ames has stated that
oxidative DNA damage is a major contributor to aging and to cancer. He points
out that any agent causing chronic cell division can be indirectly mutagenic
because it increases the probability of endogenous DNA damage being
converted to mutations. If chemicals are administered at doses substantially
lower than MTD they are not likely to cause elevated rates of cell death and cell
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