Answer to Question #910 Submitted to "Ask the Experts"

Q

Do the unpaired electrons that sometimes occur from x rays act upon the body in the same way that free radicals do? If so, does this mean that less radiosensitive tissue (muscle, neurons) is also less sensitive to free radicals? Also, why is some tissue less radiosensitive? Is it because it requires a higher concentration of unpaired electrons to upset a cell's chemistry?

A

Let me cover a few basic ideas that might help with your first question. A "free radical" is an atom that has an unpaired electron. Radiation, including x rays, does produce free radicals, ions, and free electrons when interacting with tissue and cells. These products also produce peroxides and other secondary chemicals. I think many of your questions will be answered if you read an explanation of how radiation causes these products and how these products cause damage. The Biological Effects of Ionizing Radiation Committee V report is online, with a very nice explanation of what goes on when ionizing radiation (like x rays) interacts with cells. Try reading the Radiation Chemical Effects section. If you go back a few pages (starting on page 9), it also gives some of the basic radiation physics involved. With regard to your second question, it makes sense that less radiosensitive tissues would be less sensitive to free radical damage, if we are talking about the same damage endpoint, damage site, and concentrations of damaging agents. Your third question is really a complicated question. Radiosensitivity is based on several factors, including the stage of the cell cycle, cell age, oxygen content, state of repair mechanisms, amount and type of radiation, amount of damage, genetic susceptibility/stability, cell differentiation, and presence of other protecting chemicals. In many cells, the cell cycle and what stage the cell is in are the major factors in radiosensitivity. Certain cell cycle stages (i.e., stages M and G2) are more sensitive to radiation than others (i.e., stage S), so cells that spend more time in the sensitive stages are more sensitive to permanent damage. Cells that are actively dividing, such as those found in bone marrow or intestinal lining, will then be more sensitive than nondividing cells like nerve cells because of the chance of catching the cells in a sensitive part of the cell cycle is greater. In answer to your last question, radiation or unpaired electrons do not really "upset a cell's chemistry" per se, because the ions and radicals tend to be made in very small amounts and they are very short lived. Radiation causes short-lived free radicals and ions that damage specific targets in the cell. The main target we consider in cells for damage is the DNA, though other areas like cell walls may be important also. The free radicals and ions may cause damage to the structure of the DNA and so inhibit the cell cycle or cell function. I hope this helps. The branch of biology that deals with this topic is called radiobiology and, if you are interested, there are several good books with much more detail in them that you can probably find at your local university library or book store. I suggest Radiobiology for the Radiologist by Eric J. Hall, published by J.B. Lippincott Company, but there are several good references available on the topic. Because of changes in how we understand cellular mechanisms, I would also suggest that any radiobiology book you read be published or revised after 1990, the later the better. Bruce Busby RSO, Genentech