A Comparison of 2-Hydroxyestradiol and U-0521 (3'4'-Dihydroxy-2-methylpropiophenone, Upjohn) as In Situ and In Vitro Inhibitors of Tyro sine Hydroxylase

Abstract: Feedback inhibition of tyrosine hydroxylase by catechols was evaluated using in situ and in vitro enzyme assays. The three catechol compounds used were norepinephrine, 2-hydroxyestradiol, and 3'4'-dihydroxy-2-methylpropiophenone (U-0521, Upjohn); representing endogenous catechol-amines, catechol estrogens, and a synthetic catechol, respectively. The in situ experiments were performed with dissociated retinal cells from rats and with stationary phase adrenergic-like neuroblastoma cells (N1E-115). The catechol estrogen, 2-hydroxyestradiol, resembled the endogenous catecholamines in its potency to inhibit in vitro and in situ tyrosine hydroxylations with IC₅₀ values of 10 μM in vitro and 100 μM in situ. The drug U-0521, which has been used as an inhibitor of catechol- O -methyltransferase (COMT), was also found to be an inhibitor of tyrosine hydroxylase. Further, it was shown to be more potent than the natural catechols, both in vitro and in situ , with IC₅₀ values of 30–600 nM.