Valyl-tRNA synthetase from chick embryo brain. Properties of the sulfhydryl groups.

The number of thiol groups in valyl-tRNA synthetase [L-valine tRNA ligase (AMP) E.C.6.1.1.9] isolated from chick embryo brain was determined. Titration with 5.5'-dithiobis(2-nitrobenzoic) acid showed 11 free SH groups calculated on the basis of a molecular weight of 110000 daltons. In 8M urea reaction with DTNB revealed 1 additional SH-group. Binding of substrates to the active site of the molecule decreased the number of titratable SH-groups. The modification of enzyme activity was studied by the use of thiol reagents: metal ions Ag+, Cu2+, Hg2+ and pCMB. The ATP-PPi exchange activity of valyl-tRNA synthetase was significantly less inhibited by metal ions than the aminoacylation activity. SH-groups essential for tRNA acylation were not required for the activation of valine. After gamma irradiation of valyl-tRNA synthetase the number of SH-groups diminished parallel with the decrease in enzyme activity.