Evidence for a regulatory role of a phosphorylation-dephosphorylation cycle in hypothalamic and lung histidine decarboxylase (HisDC) activity

Abstract

To investigate the modulation of histidine decarboxylase (HisDC) activity by the degree of phosphorylation of a modulatory (or enzyme) protein, HisDC was partially purified from the cytosol of the hypothalamus, the lungs and the glandular stomach of rats by ammonium sulphate precipitation (25–45% saturation) and incubated with highly purified phosphoprotein phosphatase (Type 2-A) under control and phosphorylating conditions by adding ATP, MgCl2, cAMP and purified protein kinase A to reaction mixtures. The presence of phosphoprotein, phosphatase 2A in reaction mixtures resulted in a partial reversal of HisDC inhibition induced by phosphorylating conditions, and a markedly enhanced basal enzyme activity in control conditions. In contrast, the glandular stomach HisDC failed to response strikingly either to phosphorylating or to dephosphorylating conditions. In the cytosol, the time-course of preincubations 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 2A. The addition of NaF (a common inhibitor of phosphoprotein phosphatases) to the preincubation mixtures resulted in instant, marked and long-lasting decreases in HisDC activity.