[Chemical modification of tryptophan residues of leucyl tRNA synthetase by N-bromosuccinimide and 2-hydroxy-5-nitrobenzyl bromide].

The structural accessibility of tryptophan residues in leucyl-tRNA synthetase from cow mammary gland has been studied using chemical modifications by N-bromosuccinimide and 2-hydroxy-5-nitrobenzyl bromide. The modifications were monitored by UV absorbance and intrinsic fluorescence of the enzyme's tryptophan residues. Under native conditions, at pH 7,8, only two exposed tryptophan residues are modified in each subunit of the dimeric enzyme. Under denaturing conditions, in 6 M guanidine hydrochloride solution, internal tryptophan residues are also modified as a consequence of unfolding of the native tertiary structure of the enzyme. Modifications of tryptophan residues resulted in inactivation of leucyl-tRNA synthetase both in aminoacylation and ATP-PPi exchange reactions. In the specific complex of leucyl-tRNA synthetase with the cognate tRNALeu one of exposed tryptophan residues is protected by tRNALeu and is not modified by the above reagents.