[Sulfhydryl groups of leucyl-tRNA synthetase from the cow mammary gland].

P-chloromercuribenzoate (PCMB) inhibition of the leucyl-t-RNA-synthetase (LeuRS) activity was studied both in the tRNA-aminoacylation and ATP-[32P]-pyrophosphate exchange reactions. It is established that the enzyme contained sulphydryl (SH) groups essential for catalytic activity. No protection of substrates from the PCMB action was observed in both reactions. It is shown that SH-groups essential for tRNA-aminoacylation were of no use for leucine activation. The PCMB modified enzyme was reactivated (80%) after the treatment with dithiothreitol. The quantity of cysteine residues in the native and denatured forms of LeuRS was determined.