Radioactive Flesh

"Probably the most useful tool for research since the discovery of the microscope" was described in detail last week by Dr. Joseph G. Hamilton of the University of California. The tool he referred to is the use of radioactive elements, and his comparison was apt. Whereas the microscope makes visible aspects of living tissue which cannot be seen with the naked eye, the use of radioactive elements as tracers makes it possible to learn what becomes of elements taken into a living organism as food, to study the intricate mechanics of metabolism in living plants and animals.

This tool has been in the hands of science only a short time. Only in 1934 did Irene Curie* and her husband, Frederic Joliot, first make ordinary elements such as iron and iodine radioactive so that they give off sub-atomic particles and gamma rays just as radium does. The invention of the cyclotron, Ernest Orlando Lawrence's great atom-smashing machine in California, simplified the manufacture of such elements so that they are now commonplace in physical laboratories. And in Copenhagen in 1935 O. Chiewitz and G. Hevesy first used such artificial radioactive elements as tracers in biological research.

In the abstruse Journal of Applied Physics, Dr. Hamilton tells how since then "the radioactive isotopes of 21 elements have already been employed . . , as tracers for metabolic studies." Their usefulness is that: 1) chemical and physiological properties of radioelements are identical with those of stable elements; 2) they can be used in tiny quantities which do not harm tissues as radium exposure sometimes injures human flesh; 3) wherever the radioelements go in organisms, they announce their presence by giving off detectable rays.

The detection of an element by its radioactivity is up to one million times as sensitive as by chemical analysis. The radioactivity can be detected by killing an animal after it has eaten radioelements, then assaying the radioactivity of its various tissues. It can also be detected by placing a Geiger counter, a device which measures radioactivity, over a part of the body where the tracer elements are suspected so that the metabolic ebb & flow of the tagged substance can thus be observed while the subject is alive. Or the detection can be accomplished by taking paper-thin slices of plant or animal tissue, placing them on a photographic plate, and letting them leave their own radio-autograph (see cut, p. 65).

Some applications to date of the new tool:

> Most thorough work so far has been done with radio-phosphorus. Hevesy and others have found that phosphorus turnover (metabolism) is slowest in the brain, somewhat faster in muscles and other organs, fastest in bones (which use 75% of the body's phosphorus). Since 1938 doctors have been using radio-phosphorus instead of radium or X-ray exposure in the treatment of leukemia, a mysterious cancerlike disease of the blood and blood-forming tissue such as bone marrow. This is the first therapeutic application resulting from tracer studies with radioelements.

> Insect metabolism is on such a minute scale that it frustrated biochemists until radioactive tracers came along. But scientists are now discovering what goes on inside moths and flies, and Hamilton suggests that important new insecticides will soon be invented when insects' weakest spots are traced down.

> By feeding plants with radioactive elements, California botanists have found to their surprise that plants can assimilate food downward as well as upward. More surprising was their discovery that "if barley plants are exposed to radio-nitrogen, a small quantity of labeled nitrogen compounds are formed in the tissues of the plants." This may well show, says Hamilton, that leguminous plants (clover, peas, etc.) are not unique in their power to fix atmospheric nitrogen.

> Most surprising radio-botanical find: chlorophyll in green plants continues to assimilate and reduce carbon dioxide from the air on a "small but measurable" scale, even in complete absence of light. This lightless "photosynthesis" demands that biochemists radically revise their theories about what is the most important chemical reaction on earth.

* Daughter of the late, great Marie Curie and, like her mother, winner of a Nobel Prize (1935) for work on radioactivity.

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