Factors Affecting thein VitroInactivation of Aldolase by X-rays

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Abstract
The influence of urea and of various protective compounds on the ‘in vitro’ inactivation of aldolase by x-rays has been studied. Low concentrations of urea protect the enzyme from inactivation, whereas high concentrations, able to induce an unfolding of the protein molecule, increase the degree of inactivation by a given radiation dose. Cysteamine, cystamine, S-(2-aminoethyl) isothiouronium and reduced glutathione all protect the aldolase in solution from inactivation by x-rays. Cystamine is as protective as cysteamine, in equimolecular concentrations, when high inactivation levels are reached. No protection can be demonstrated when the aldolase after incubation with the tested compounds is precipitated and re-dissolved in a new medium before irradiation. Nevertheless, with 35S-labelled cystamine, it can be demonstrated that at least seven residues of cysteamine are bound to each aldolase molecule. The protective power of glutathione is reduced by a factor of about 0·2 in the presence of 4 M urea. The possible implications of these findings are discussed.