A Kinetic Study of Bovine Adrenal Tyrosine Hydroxylase

From the ¹ From the Laboratory of Clinical Biochemistry, National Heart Institute, National Institutes of Health, Bethesda, Maryland 20014

Kinetic data are presented which are consistent with the concept that the mechanism of tyrosine hydroxylase involves the following sequential steps: (a) reduction of an oxidized form of the enzyme by 2-amino-4-hydroxy-6,7-dimethyltetrahydropteridine (DMPH₄) followed by dissociation of the oxidized pteridine, and (b) aerobic oxidation of tyrosine, producing 3,4-dihydroxyphenylalanine (dopa) and the oxidized form of the enzyme. This concept is supported by studies of the effects of the substrates on initial velocities as well as by the use of dopa and analogues of substrates and products as inhibitors. Tyrosine and its analogues bind to a reduced form of the enzyme, while dopa and its analogues bind to an oxidized form of the enzyme. Deviations from this pattern occur at concentrations several times the Michaelis constant. At such concentrations tyrosine and its analogues can be bound to an oxidized form of the enzyme, while dopa and its analogues and DMPH₄ can be bound to a reduced form of the enzyme.

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