Enhancement of rat brain catecholamine synthesis by administration of small doses of tyrosine and evidence for substrate inhibition of tyrosine hydroxylase activity by large doses of the amino acid.

Rat brain catecholamine synthesis is enhanced by small doses of tyrosine, but not by doses of 50mg/kg body wt. and above. It is suggested that these latter doses overcome the above enhancement by causing a substrate inhibition of tyrosine hydroxylase activity.