The dithiol, cystamine, affords marked protection against the effects of X-irradiation on potassium leakage from baker's yeast cells. Little protection is afforded under the same conditions against irradiation-induced inhibition of colony formation. Analysis of the binding of cystamine to the cell surface shows that the agent does not penetrate to the cell interior. This is considered to account for the lack of protection of cell growth mechanisms. Furthermore, the cystamine can be largely displaced from the cell surface by competition with the bivalent cations, manganous and calcium. Yet, the radioprotection against changes in K+ leakage rates was identical whether 99% or only 30% of the binding sites were occupied by cystamine. This suggests that its mode of action may be mediated largely by radical scavenging in the medium or immediate environs of the membrane, and establishes that binding is not essential for radioprotection in this system.

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