Changes associated with early weaning in the activity of tyrosine hydroxylase in the caudate nucleus of the piglet.

1. Early weaning in piglets is associated with changes in dopamine metabolism in the caudate nucleus. In particular, a decrease in the amount of the enzyme tyrosine hydroxylase was observed. 2. An analysis of the kinetic properties of tyrosine hydroxylase from the caudate nucleus of early-weaned piglets shows that there is a relative increase in the activity of the enzyme which may indicate a mechanism compensating for the decreased amount of enzyme. 3. The increase in activity is mediated through a decrease in the Km for tetrahydrobiopterin and is reversed, in vitro, when the tissue is homogenized at pH values greater than 8.0.