DNA radiation damage and its modification by metallothionein.

Thiol compounds have long been known to protect living cells against the harmful effects of ionizing radiation. Maetallothionein is a naturally occurring low molecular weight polypeptide rich in cysteine residues and may be useful in protection against low-level radiation effects. Radiation damage to DNA and its nucleotide components and the radioprotective effect of metallothionein have been studied in model chemical systems and compared to its effect on cells. Metallothionein acts both as a free radical scavenger and a reductant, and its radioprotective effectiveness has been studied as a function of dose, drug concentration, and in the presence and absence of oxygen. It is more effective in protecting against sugar-phosphate damage under hypoxic conditions. The chemical modification is greater than that of cell killing as measured by the loss of colony-forming ability. Dose reduction factors greater than two are observed for DNA radioprotection, but the values in cells are much lower. These findings will be discussed in terms of the molecular mechanisms and their implications.