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The transport of norepinephrine and two key enzymes involved in its synthesis, tyrosine hydroxylase (EC 1.14.3a) and dopamine beta-hydroxylase (EC 1.14.2.1), has been studied in relation to other axonal constituents in ligated chicken sciatic nerves. Norepinephrine, tyrosine hydroxylase, and dopamine beta-hydroxylase activity all increased proximal to the constriction over a 20-hr period. The rate of transport of norepinephrine, tyrosine hydroxylase, and dopamine beta-hydroxylase were calculated as 2.6, 1.9, and 3.4 mm/hr, respectively. The only enzyme marker to show a similar rate of accumulation was acetylcholinesterase (EC 3.1.1.7), a putative marker for endoplasmic reticulum. The rapid axoplasmic transport of tyrosine hydroxylase from the cell bodies to the terminals of sympathetic neurons may be adequate to account for the elevated amounts of this enzyme in chronically active axon terminals, without the necessity to invoke peripheral axonal synthesis of the enzyme to explain such elevated amounts.

1. The question of cellular localizations of monoamine oxidase and catechol-o-methyl transferase, enzymes involved in the metabolism of noradrenaline, has been investigated by following the changes in the enzyme activity of the smooth muscle of the cat nictitating membrane after sympathetic denervation. Any falls in enzyme activity coinciding with the time course for degeneration of the sympathetic nerve endings (2-3 days) can reasonably be ascribed to a former localization of the enzyme within the sympathetic nerve.2. Monoamine oxidase activity of the inferior and medial smooth muscle was significantly reduced 3 days after sympathectomy and remained reduced 14 days after sympathectomy.3. A correlation was found between the fall in monoamine oxidase activity and the endogenous noradrenaline concentration of the contralateral normal muscle, indicating that the higher the endogenous noradrenaline concentration, the greater the fall in monoamine oxidase activity after denervation.4. It was concluded that the fall in monoamine oxidase activity after denervation indicated a former localization of the enzyme within the sympathetic nerve endings.5. The fall in monoamine oxidase activity of the denervated muscle was significantly less measured with benzylamine as a substrate than with tyramine. This suggested that the neuronal monoamine oxidase may have different properties than the extraneuronal enzyme.6. A small fall in the catechol-o-methyl transferase activity of denervated smooth muscle was found and a correlation between the fall in enzyme activity and endogenous noradrenaline indicated that in this smooth muscle a proportion of the catechol-o-methyl transferase activity may be of presynaptic origin.