Effect of 60Co gamma radiation on the valyl-tRNA synthetase isolated from chick embryo brain.

The effect of 60Co gamma irradiation on the activity of valyl-tRNA synthetase isolated from chick embryo brain was studied. The enzyme activity exponentially decreased in the dose range 10--200 krad. The first step of the enzyme action, i.e. the amino acid activation, was found to be less sensitive to irradiation than the whole reaction, the formation of valyl-tRNA, 2-Mercapto ethanol and/or glycerol had a significant radioprotective effect. The lesion caused by radiation in the enzyme was also influenced by its concentration during exposure (dilution effect). According to gel-electrophoretic experiments, no chain rupture occurred in the enzyme molecule. Not even a change in Km was observed; however, the maximum velocity of the reaction was found to decrease with increasing radiation dose.