Cysteamine: differential x-ray protective effect on Chinese hamster cells during the cell cycle.

Cysteamine present during x-irradiation protects synchronized Chinese hamster cells in culture against lethal damage at all stages of the cell cycle. The effect is greatest for cells irradiated at sensitive stages such as G(1) and least for resistant cells; for example, late S (dose-modifying factors 4.2 and 2.7, respectively). The effect of 50 millimolar cysteamine is to render almost invariant the normally variant x-ray age response for lethality. This suggests that there are two components of x-ray damage, only one of which is age dependent, and it is against this component that cysteamine protects the cell. Cystamine, however, has no protective effect upon these cells at any stage of the cell cycle.