Effect of irradiation on the GTP binding kinetics of chicken embryo brain plasma membranes.

The membrane bound form of the catalytic subunit of adenylate cyclase of chicken embryo brain has been found earlier to be rather radioresistant [28]. The radiation induced changes of G proteins of membrane preparations of 19 day old chicken embryo brains were investigated in this work. The activation of catalytic subunit of adenylate cyclase by G protein dependent activators (Gpp/NH/p and NaF) was found elevated at lower radiation doses (0-400 Gy), while both basal enzyme activity (measured without any activator) and the activity measured in the presence of activators decreased at higher doses (above 800 Gy). Heterogeneity of GTP binding sites measured with 3-H-Gpp/NH/p a GTP-ase resistant GTP analogue was observed in the case of control (with Kd1 = 0.0663 +/- 0.034 mumol/l, Bmax = 0.0079 +/- 0.0022 nmol/ml and Kd2 = 2.038 +/- 0.4779 mumol/l, Bmax2 = 0.0291 +/- 0.0017 nmol/ml). The lower affinity high capacity binding sites seemed to be more radiosensitive, than the higher affinity sites. A marked decrease was observed in the number of low affinity binding sites above 200 Gy and these low affinity binding sites practically disappeared after irradiation with 400 Gy. At high doses (above 1600 Gy) the catalytic subunit was damaged, too. On the basis of the decrease of low affinity binding sites together with an increase in activation of the catalytic subunit via G proteins one can conclude that it is caused by radiation induced damage of Gi protein that can be more radiosensitive, than Gs protein.