EVIDENCE FOR A REGULATORY ROLE OF A PHOSPHORYLATION-DEPHOSPHORYLATIONCYCLE IN HYPOTHALAMIC AND LUNG HISTIDINE-DECARBOXYLASE (HISDC) ACTIVITY

To investigate the modulation of histidine decarboxylase (HisDC) activ ity by the degree of phosphorylation of a modulatory (or enzyme) prote in, HisDC was partially purified from the cytosol of the hypothalamus, the lungs and the glandular stomach of rats by ammonium sulphate prec ipitation (25-45 % saturation) and incubated with highly purified phos phoprotein phosphatase (Type 2-A) under control and phosphorylating co nditions by adding ATP, MgCl2, cAMP and purified protein kinase A to r eaction mixtures. The presence of phosphoprotein phosphatase 2A in rea ction mixtures resulted in a partial reversal of HisDC inhibition indu ced by phosphorylating conditions, and a markedly enhanced basal enzym e activity in control conditions. In contrast, the glandular stomach H isDC failed to response strikingly either to phosphorylating or to dep hosphorylating conditions. In the cytosol, the time-course of preincub ations showed an increased HisDC activity during the first 1 - 20 min incubation period and more enhanced enzyme activity in the presence of MnCl2, a stimulator of phosphoprotein phosphatase 2 A. The addition o f NaF (a common inhibitor of phosphoprotein phosphatases) to the prein cubation mixtures resulted in instant, marked and long-lasting decreas es in HisDC activity. All these data indicate that HisDC activity in t he hypothalamus and the lungs (but not in the stomach) is under the re gulation of a phosphorylation-dephosphorylation cycle. Moreover, the e nzyme activity is depended on the degree of phosphorylation.