Tyrosine hydroxylase: activation by nerve stimulation

Abstract

The synthesis of the sympathetic neurotransmitter, norepinephrine, is accelerated by electrical stimulation of the guinea pig vas deferens. The molecular mechanism responsible for this enhanced formation of transmitter is unknown but has been attributed to an increase in the activity of tyrosine hydroxylase (EC 1.14.16.2; tyrosine 3-monooxygenase) during nerve stimulation. In the present experiments, we found that crude preparations of tyrosine hydroxylase isolated from guinea pig vasa deferentia that were electrically stimulated or depolarized by potassium show an increase in activity compared with enzyme obtained from untreated paired control tissues. This increase in activity is partially antagonized by addition of the Ca(++) chelator, ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA), to the assay medium, and can be completely blocked if Ca(++) is removed from the potassium-rich medium used to depolarize the intact tissue, before preparation of the enzyme. A similar increase in enzyme activity occurs when Ca(++) ions are added directly to enzyme prepared from untreated vasa deferentia. In this instance, the activation is completely reversed by EGTA. The increase in activity produced by addition of Ca(++) to the isolated enzyme or by electrical stimulation or potassium depolarization of the tissue before isolation of the enzyme appears to be mediated by changes in the kinetic properties of tyrosine hydroxylase. All treatments appear to activate tyrosine hydroxylase by causing an increase in its affinity for substrate and pteridine cofactor and by decreasing its affinity for the end-product inhibitor, norepinephrine. These results provide direct evidence that the enhanced formation of norepinephrine seen during stimulation of sympathetically innervated tissues arises from an activation of tyrosine hydroxylase. The fact that the activation produced by nerve stimulation is mimicked by Ca(++) ions raises the intriguing possibility that the influx or mobilization of Ca(++) that accompanies nerve stimulation and that is intimately involved in release of transmitter may also participate in the activation of tyrosine hydroxylase.