Figure 1.

Inhibition of PP-1C by dephospho- and phospho-DARPP-32. PP-1C was assayed using 10 μM [³²P]phosphorylase a as substrate. (A) Phospho-DARPP-32 (•), phospho-D32[1–38c] (▴), DARPP-32[T34A] (○), and dephospho-D32[1–38c] (▵) were present in the indicated concentrations. (B). Mechanism of inhibition of PP-1C by nonphosphorylated-DARPP-32. (Left) Lineweaver–Burk analysis at various concentration of DARPP-32[T34A]. PP-1 activity was determined using a range of concentrations of [³²P]phosphorylase a (2.5–20 μM), in the absence (▪) or presence of dephospho-DARPP-32 (□, 0.5 μM; •, 1.5 μM; ○, 4 μM; ▴, 8 μM). (Right) Secondary replot of K_m/V_max as a function of DARPP-32 concentration.