Radioassay of TaK activity. 1,3-14C-Mal-S-TaB (filled squares), 1,3-14C-Mal-S-TaE (open squares), and 1,3-14C-Memal-S-TaE (open circles) were incubated with TaK. At several time points, reaction aliquots were withdrawn and quenched by addition of trichloroacetic acid, and residual protein-linked radiolabel was measured by liquid scintillation counting. Loss of protein-linked radiolabel is associated with generation of 1-14C-Ac-S-TaB from 1,3-14C-Mal-S-TaB with concomitant evolution of 14CO2. Each point represents the average of three trials.