Release of Thiols from Cellular Mixed Disulphides and Its Possible Role in Radiation Protection

Abstract
The non-protein-bound part of cellular mixed disulphides between proteins and thiols was analysed by gel-filtration after reduction of the disulphide bonds. Glutathione (GSH) was found to be the only detectable low-molecular thiol bound to cellular proteins. Reduction of disulphide bonds resulted, in addition to the release of GSH, in the liberation of sulphydryl-containing protein fragments of various sizes. Using cells in which the sulphur was labelled with 35S, it was demonstrated that about 15 per cent of the incorporated cysteamine was bound to the proteins, and that GSH was released in comparable amounts. The protein-cysteamine bound was stable for at least 95 min. It is concluded that GSH is released from proteins by cysteamine treatment in an exchange reaction. The data suggest, furthermore, that protein-bound GSH may be regarded as a reservoir of a radioprotective substance which can be released under certain conditions and, when released, can contribute to enhance the radioresistance of cells.