Studies on the valyl-h-tRNA synthetase of chick embryo brain irradiated with 60Co gamma-rays, in vivo.

18-day-old chick embryos (Leghorn) were irradiated in vivo with 1,2,3,4,5 or 7 Gy 60Co gamma-rays (dose rate = 0.9 Gy/min). Twenty-four hours after irradiation the activity of valyl-tRNA synthetase isolated from the brains was determined and compared with that of the non-irradiated control. The aminoacylation activity was found to decrease exponentially as a function of the dose (D37 = 6 Gy). Irradiation caused a more pronounced decrease in valyl-tRNA synthetase activity in 18-day-old than in 14-day-old chick embryos and induced in the valine-dependent ATP-PPi exchange reaction less change than in the valyl-tRNA FORMATion. The exposure of 15-day-old chick embryos to a dose of 4 or 5 Gy induced, respectively, a 40 or 20 per cent increase in enzyme activity of VRS (prepared on day 19 of embryonic life) relative to the control. In these experiments a decrease of Km value for tRNA has been found. The change in the number of sulfhydryl groups was also investigated.