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

Abstract

(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.