Limited proteolysis of rat brain tyrosine hydroxylase defines an N-terminal region required for regulation of cofactor binding and directing substrate specificity.

Trypsin proteolysis of tyrosine hydroxylase (TH) produces a 34-kDa fragment that is catalytically active but does not contain the regulatory phosphorylation sites. In this report, activation of TH by proteolysis was characterized further. Proteolysis results in a decrease in Kms for both substrate and cofactor. The increase in affinity for cofactor was identical to that produced by phosphorylation with cAMP-dependent protein kinase. Additionally, proteolysis of an N-terminal region containing the regulatory phosphorylation sites was sufficient to produce a decrease in Km for cofactor. Activation of substrate binding required more extensive proteolysis but also corresponded to N-terminal digestion. Moreover, this activation was coincident with a broadened substrate specificity. In combination, these data indicate that the N-terminus of tyrosine hydroxylase regulates cofactor binding and directs substrate specificity.