Models for thiol protection of DNA in cells

Abstract

It is generally assumed that death of a cell from exposure to ionizing radiation results primarily from damage to DNA, although the exact relationship between the two endpoints is not clear. Furthermore, the relationship between modification of DNA damage by the addition of thiols and modification of cell killing is not understood. Various models have been used to study DNA damage and the effects of thiols on it. These include studies of radiation chemistry of DNA constituents, physical damage such as strand break and crosslink production in DNA irradiated in solution, biological activity of DNA irradiated in solution, and physical damage in DNA from irradiated cells. It is difficult to compare the results obtained in these various studies and to extrapolate from them to cell killing, in part at least, because very different radiation conditions (e.g. dose, dose rate, buffer, pH, thiol compound, etc.) are often used. Over the past several years we have performed studies with thiols using a number of systems: pulse radiolysis of DNA constituents (Held et al., 1985), biological activity of bacterial transforming DNA (Held et al. , 1981, 1984a,b), DNA strand breaks in mammalian cells (Held et al., 1986), and mammalian cell killing (Held, 1985). In all these studies the thiol compound dithiothreitol (DTT) was used, and this should help in making comparisons and extrapolations from one system to the next. In this paper these studies will be reviewed and compared with other relevant literature in an attempt to address the question: Is DNA the 'target' for thiol modification of radiation-induced cell killing? A number of different hypotheses have been proposed to account for radioprotection by thiolcontaining compounds. These have been listed and discussed previously (e.g. Klayman and Copeland, 1975) and include OH radical scavenging by thiols; hydrogen atom donation from thiols to radiation-induced organic free radicals in competition with reaction of those radicals with oxygen [the repair-fixation model (Alexander and Charlesby, 1955)]; hypoxia induction by oxidizing thiols (e.g. Durand, 1983); chelation of metals; and production of disulfides. The second question to be addressed is whether DNA protection can be explained by these various hypotheses in relation to the data presented, with particular emphasis given to the first two hypotheses. This review is limited to discussion of initial damage induced in DNA by ionizing radiation; discussion of biochemical or enzymatic repair processes is beyond the scope of this paper. Furthermore, only effects on DNA itself will be discussed; although membranes or the DNAmembrane complex may be involved in radiation-induced cell killing (Alper, 1979; Patrick, 1977), these effects will not be addressed.