Identification of protein phosphatase 2A as the major tyrosine hydroxylase phosphatase in adrenal medulla and corpus striatum:evidence from the effects of okadaic acid

(i) The major sites on bovine adrenal tyrosine hydroxylase (TH) phosphorylated by calmodulin-dependent multiprotein kinase (CaM-MPK) and cyclic AMP-dependent protein kinase were shown to be Ser-19 and Ser-40, respectively, while Ser-40 was also phosphorylated slowly by CAM-MPK. (ii) Type 2A and type 2C phosphatases accounted for ≈90% and ≈ 10% of TH phosphatase activity, respectively, in extracts of adrenal medulla and corpus striatum assayed at near physiological free Mg2+(1 mM), while type 1 and type 2B phosphatases had negligible activity towards TH. (iii) Incubation of adrenal chromaffin cells with okadaic acid increased TH phosphorylation by 206% and activity by 77%, establishing that type 2A phosphatases play a major role in regulating TH in vivo.