Dopaminergic neurons: activation of tyrosine hydroxylase by a calcium chelator
Morgenroth, V.H.; Boadle-Biber, M.C.; Roth, R.H.
Molecular Pharmacology 12(1): 41-48
Addition of the Ca2+-chelating agent ethylene glycol bis(.beta.-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) to a high-speed supernatant preparation obtained from rat striatum produced a dramatic increase in the activity of tyrosine hydroxylase assayed in the presence of subsaturating concentrations of tyrosine and reduced pterin cofactor. This activation appeared to be mediated by changes in the kinetic properties of tyrosine hydroxylase. In the presence of EGTA (50 .mu.M) the Km of the enzyme for tyrosine decreased nearly 8-fold (from 54 .mu.M to 9 .mu.M), the Km for 2-amino-4-hydroxy-6,7-dimethyl-5,6,7,8-tetrahydropteridine decreased 7-fold (from 0.89 mM to 0.13 mM), and the Ki for dopamine increased 700-fold (from 0.11 mM to 74 mM). No significant change in Vmax was observed. All these kinetic alterations could be reversed by the addition of Ca2+ to the assay medium, while Mg2+, even in high concentrations (1 mM), was ineffective. A similar activation was observed with tyrosine hydroxylase isolated from other dopamine-containing regions of rat brain (median eminence, olfactory tubercle) and in the dopamine-rich pedal ganglion of Mercenaria mercenaria (Mollusca). EGTA produced no significant change in the activity of tyrosine hydroxylase prepared from central (medulla-pons) or peripheral noradrenergic neurons. Tyrosine hydroxylase associated with dopaminergic neurons may differ from the enzyme found in noradrenergic neurons. This unique response of the tyrosine hydroxylase present in dopaminergic neurons may serve as a useful biochemical marker for identifying these neurons.