Figure 12.

Re-establishing robust DOPA homeostasis at reduced TH activity levels by DOPA addition. In both panels: phase 1 has a normal TH activity ($k_2$ = $1\times {10}^4 \mathsf{\mu }$M/min) and no oxidative stress ($k_{14}$ = 0.0 $\mathsf{\mu }$M/min); in phase 2, oxidative stress is present ($k_{14}$ = 0.1 $\mathsf{\mu }$M/min), and TH activity level is reduced by two orders of magnitude ($k_2$ = $1\times {10}^2 \mathsf{\mu }$M/min). As a result, DOPA homeostasis breaks down in phase 2 with low DOPA and DA levels. In phase 3, DOPA is added ($k_{22}$ = 7 $\mathsf{\mu }$M/min) in the presence of low TH and oxidative stress as in phase 2. In phase 3, DOPA homeostasis is restored. Panel (a): $k_8$ = 0.29 $\mathsf{\mu }$M; panel (b): $k_8$ = 0.029 $\mathsf{\mu }$M. Other rate constant values and initial concentrations, as in Figure 10a.