Effect of cortisol on the thiol content of rat liver nuclear proteins

Administration of cortisol to normal or adrenalectomized rats leads within 15-30min to an increased thiol content of nuclear proteins, measured by the incorporation of iodo[(3)H]-acetate or N-[(14)C]ethylmaleimide or by colorimetric methods. The same effect is observed after incubation of isolated rat liver nuclei with corticosteroids. The increased thiol content of the nuclear proteins shows the same time-dependence as the stimulation of RNA synthesis by corticosteroids observed in vivo and in vitro. Amino acid analysis of the carboxymethylated proteins reveals that in the experiments in vivo most of the label is present as carboxymethylcysteine with small amounts of carboxymethyl-lysine and carboxymethylhistidine, whereas in the experiments in vitro more carboxymethyl-lysine and carboxymethylhistidine than carboxymethylcysteine are found. The increase in the content of thiol groups is due to cleavage of the disulphide bridges between the nuclear proteins. Polyacrylamide-gel electrophoresis of the acid-soluble fraction reveals that most of the iodo[(3)H]acetate label is incorporated into a non-histone fraction with a molecular weight of approx. 45000 whereas in the acid-insoluble fractions many protein bands are labelled.