Fig. (6) Stoichiometry of labeling of IleRS by ADP-PL.IleRS (6 µM) was incubated with 0.2-3 mM ADP-PL, as described under Methods. The residual enzyme activity is plotted against the molar fraction of ADP-PL incorporated per mole of enzyme (n), as deduced from the absorbance at 325 nm of NaBH₄-reduced ADPPL covalently bound to IleRS and separated from the unbound reagent by chromatography through a TSK 4000 PW column.