Studies on a Molecular Basis for the Heparin‐Induced Regulation of Enzymatic Activity of Mouse Striatal Tyrosine Hydroxylase In Vitro. Inhibition of Heparin Activation and of the Enzyme by Poly‐L‐lysyltyrosine and Poly‐L‐lysylphenylalanine and Their Constituent Peptides

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Abstract
Tyrosine hydroxylase was purified up to 10‐fold from hypotonic extracts of mouse striatum by heparin affinity chromatography. The purified enzyme (a) had a low Km for tyrosine (around 15 μm) and was not inhibited by tyrosine at concentrations up to 0.2 mm when tetrahydrobiopterin was cofactor and (b) was activated by heparin. The interaction of heparin with tyrosine hydroxylase was studied in ways relating to the known interaction with anti‐thrombin. Heparan and keratan sulfates failed to activate tyrosine hydroxylase in place of heparin; several fractions of the bulk heparin (constituting 5 and 15%) had enriched tyrosine hydroxylase‐activating potency; and two lysine copolypeptides (polylysyltyrosine and polylysylphenylalanine) inhibited the activation of tyrosine hydroxylase by heparin. The lysine copolymers also directly inhibited the enzyme. Heparin (but not heparan and keratan sulfates) protected tyrosine hydroxylase from this inhibition. The constituent lysyltyrosyl (but not lysylphenylalanyl) peptide inhibited tyrosine hydroxylase, and heparin also reversed this inhibition, which was sigmoidal (IC50 of 490 μM) and partially competitive with tyrosine. Tyrosine hydroxylase was purified up to sevenfold by lysyltyrosyl‐affinity chromatography. This enzyme preparation exhibited an eightfold greater sensitivity to lysyltyrosylamide than tyrosine hydroxylase purified by heparin affinity. The data indicate that tyrosine hydroxylase is regulated in vitro by a negatively charged site. Occupancy of this site by cationic effectors results in allosteric inhibition which mediates changes in the apparent Km for tyrosine.

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