Ammonium ion contamination effects from deamination of L-Ser in the tryptophan synthase-catalyzed pyruvate side reaction. (A) Effects of L-Ser - α2β2 incubation on the indoline reaction (incubation times are indicated). Final concentrations: α2β2, 10 µM; L-Ser 50 mM, indoline 5 mM. (B) Effects of NH4+ preincubation with α2β2. Final concentrations: α2β2, 10 µM; L-Ser 50 mM, indoline 5 mM, and NH4+ as indicated. Time courses in (A) and (B) were measured using SWSF instrumentation. (C) Comparison of the effect of L-Ser incubation time (black circles) on amplitude with the effects of added [NH4+] (white circles) on the fast phase of the indoline reaction (α2β2 preincubated with NH4+). Amplitudes taken from (A) and (B). To correct for fast phase amplitude loss in the instrument dead-time, the total amplitude of the fast phase was estimated as the sum of the observed fast phase and the y-intercept value for the reaction of α2β2 and L-Ser, with indoline. Traces were fit to the sum of two exponentials. The NH4+ concentrations for time resolved data points (black circles) were estimated using kcat = 0.098 s−1 for pyruvate formation at each incubation time in (A).