The condensed matrix of mature chromaffin granules. The soluble form of dopamine beta-hydroxylase is catalytically inactive

Kinetic evidence for channeling of dopamine between monoamine transporter and membranous dopamine-beta-monooxygenase in chromaffin granule ghosts

The nature of coupling between the uptake and dopamine-beta-monooxygenase (DbetaM) catalyzed hydroxylation of dopamine (DA) was studied in bovine chromaffin granule ghosts. Initial rate and transient kinetics of DA uptake and conversion were determined under a variety of conditions. The uptake kinetics of DA, norepinephrine (NE), and epinephrine demonstrate that DA is a better substrate than NE and epinephrine under optimal uptake conditions. The transient kinetics of DA accumulation and NE production under both optimal uptake and uptake and conversion conditions were zero-order with no detectable lag or burst periods. The mathematical analyses of the data show that a normal sequential uptake followed by the conversion process could not explain the observed kinetics, under any condition. On the other hand, all experimental data are in agreement with a mechanism in which DA is efficiently channeled from the vesicular monoamine transporter to membranous DbetaM for hydroxylation, prior to the release into the bulk medium of the ghost interior. The slow accumulation of DA under optimal conversion conditions appears to be caused by the slow leakage of DA from the channeling pathway to the ghost interior. Because DbetaM activity in intact granules is equally distributed between soluble and membranous forms of DbetaM, if an efficient channeling mechanism is operative in vivo, soluble DbetaM may not have access to the substrate, making the catalytic activity of soluble DbetaM physiologically insignificant, which is consistent with the increasing experimental evidence that membranous DbetaM may be the physiologically functional form.

Anion- and pH-dependent activation of the soluble form of dopamine beta-hydroxylase

The present study presents evidence for the conclusion that the catalytic activity of dopamine beta-hydroxylase (DBH; dopamine beta-mono-oxygenase, EC 1.14.17.1) is regulated independently by pH and by anions. In the absence of activating anions (i.e. in 50 mM Mes buffer) the activity was essentially zero at low pH (5.1-5.3) when assayed with the artificial electron donors ferrocyanide (0.25 mM), N, N, N', N'-tetramethyl-p-phenylenediamine (TMPD, 2 mM) or N, N-dimethyl-p-phenylenediamine (1 mM) and tyramine (8 mM) as the substrate to be hydroxylated. However, in the presence of activating anions (e.g. 0.05-0.6 M Cl(-) in 50 mM Mes buffer, 0.1 M phosphate buffer or 0.2 M acetate buffer) a high catalytic activity was observed at pH 5.1-5.3. The pronounced effect of anions at this pH may be related to the postulated anion-induced conformational change of DBH [Syvertsen, Melø and Ljones (1987) Biochim. Biophys. Acta 914, 6-18] resulting in a facilitated access of the substrates to the active site(s). The anion-activated DBH was inhibited when assayed with ferrocyanide and activated when assayed with TMPD as electron donors by increasing the pH (5.1 to 6.0). By contrast, in the absence of anions the activity increased from essentially zero at pH 5.1-5.3 to high values at pH 6.0, irrespective of the electron donor used. The data suggest that the conformational change induced by anion activation exposes a negatively charged group at or near the electron-donor-binding site(s) imposing an electrostatic repulsion towards ferrocyanide (four negative charges) and an electrostatic attraction towards the positively charged TMPD, thus explaining the different pH-activity curves obtained for the two electron donors. In contrast to the artificial electron donors, the physiological donor ascorbate [Terland and Flatmark (1975) FEBS Lett. 59, 52-56] supports hydroxylation of tyramine at low pH also in the absence of Cl(-), acetate or phosphate, confirming that ascorbate also functions as an anion activator.