Rapid axonal transport of tyrosine hydroxylase in rabbit sciatic nerves

We compared the axonal transport of tyrosine hydroxylawe with that of dopamine-β-hydroxylase by means of an in vitro stop-flow technique. Proximal to a locally cooled nerve region, the activity of both enzymes increased linearly for 3 h. Internal standards of partially purified adrenal enzyme showed that the increased activity of tyrosine hydroxylase was not due to changes in activators or inhibitors. Furthermore, this increased activity represented increased V_max, unaccompanied by changes in K_m for tyrosine; probably, therefore, it reflected increased concentrations of the enzyme. From the rates of increase of enzyme activity, we calculated that, on the average, tyrosine hydroxylase was transported at 0.78 mm/h, and dopamine-β-hydroxylase at 2.4 mm/h. Much less tyrosine hydroxylase than dopamine-β-hydroxylase seemed free to move in the nerves, suggesting greater similarity between true transport velocities than between average velocities of these enzymes. When nerves were rewarmed after local cooling for 3 h, waves of accumulated enzyme activity moved distally. The wave of dopamine-β-hydroxylase activity had a single peak moving at about 14 mm/h. The wave of tyrosine hydroxylase activity had a small component moving this fast, but more of it moved slower, at velocities down to 5 mm/h. Although tyrosine hydroxylase was apparently much more soluble than dopamine-β-hydroxylase there was no evidence that the rapidly transported fraction of either enzyme had a different subcellular distribution from that of slower transported or stationary fractions.