Biological effects of hot atom reactions

Nuclear transmutations of radioactive isotopes incorporated into biological systems may be very effective in altering structure and function of vitally important organic compounds. It is especially the DNA that is considered the critical molecule for radiation effects and transmutation effects. Whereeas single-strand breaks of DNA are efficiently repaired, double-strand breaks are probably lethal events. To describe transmutation effects from incorporated isotopes it is necessary to differentially recognize radiation effects such as from emitted beta-particles in the course of nuclear decay. For this purpose microdosimetric approach to analysing dose-effect relationships is necessary. A review is given on transmutation effects in DNA from 3H, 32P, 33P, 14C, and 125I. Special emphasis is placed on the biological consequences of the Auger effect produced by the dacay of 125I. An analysis of the data indicates that the number or radiation-induced DNA double-strand breaks produced per decay of the incorporated nuclides exceeds the number of transmutation-induced DNA double-strand breaks by a factor of more than 200 for 32P randomly distributed in tissue. This factor is of the order of 40 for 14C randomly distributed in tissue; yet it is 3 for 14C localized only in cell nucleus including its DNA. For 3H exclusively incorporated into nuclear DNA the factor is expected to be approximately 8. In the case of 125I decay, the transmutation effect is calculated to be at least twice as toxic as the effect expected on the basis of absorbed radiation. Yet experimental evidence indicates that decay of 125I in DNA may be approximately 10-100 times more effective in inactivating cells and phage than is decay of 3H in DNA. This discrepancy between experimental evidence and calculated data indicated that the ''anatomy'' and repair of the DNA double-strand breaks produced by the Auger effect is different from that caused by irradiation. (author)