Effects of Ovarian Steroids on in vitro Kinetic Properties of Tyrosine Hydroxylase from Rabbit Oviducts1

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Abstract
The kinetic properties of tyrosine hydroxylase (TH) isolated from oviductal isthmuses of rabbits in different hormonal states were studied in order to determine whether the changes in norepinephrine (NE) metabolism in the oviduct observed previously were related to alterations in the activity of this rate limiting enzyme for NE synthesis. Ovariectomy reduced the reaction velocity (Vmax) of both TH and its pteridine cofactor, but did not change the affinity (Km) of the TH for either substrate or cofactor. Treatment of the castrates with estrogen or estrogen and progesterone returned Vmax to normal estrous levels. The increase in NE content and turnover found previously in oviducts from castrates given ovarian steroids is related to an elevation of Vmax for both TH and cofactor and not to a change in Km. These actions of the ovarian steroids are like of those described for the effects of adrenocorticotropins on TH activity of the adrenal gland of hypophysectomized rats and represent another example of the influence of hormones on transmitter metabolism in the adrenergic nervous system.