170 CONTROL MECHANISMS IN CELLULAR PROCESSES 



sion of Talalay and Williams-Ashman ( 1958 ) that this hormone 

 stimulated the transfer of hydrogen from TPNH to DPN. 



Reaction Mechanism 



Estradiol-17yS and estrone are equally effective in promoting the 

 TPNH-DPN transhvdrogenase reaction in soluble placental extracts. 

 Partially purified enzyme preparations were found to catalyze the 

 reduction of DPN and TPN by estradiol- 17/3, and the oxidation of 

 DPNH and TPNH by estrone (Talalay and Williams-Ashman, 1958). 

 This suggested tliat these steroids stimulated the transfer of hydro- 

 gen from TPNH to DPN by undergoing alternate oxidation and 

 reduction, as follows: 



Estrone + TPNH + H+ ^ Estradiol-17/5 + TPN+ 

 Estradiol-17/5 + DPN+ ^ Estrone + DPNH + H + 



TPNH + DPN+ ^ TPN+ + DPNH 



According to this reaction mechanism, these steroids transport hy- 

 drogen in a coenzyme-like manner by yirtue of the change steroid 

 alcohol ^ steroid ketone. 



Previously, Langer and Engel (1958) characterized a placental 

 17yS-hydroxy steroid dehydrogenase which reacted with a number of 

 1, 3, 5-estratriene derivatives including estrone and estradiol-17/3. 

 Both DPN and TPN served as hvdrogen acceptors for the oxidation 

 of estradiol-17/3. Talalay and Williams-Ashman (1958, 1960) have 

 provided evidence tJiat the estradiol-mediated transhvdrogenase re- 

 actions in placenta are catalyzed by an enzyme which exhibits the 

 properties of the 17^-hydroxysteroid dehydrogenase of Langer and 

 Engel ( 1958 ) . The enzyme catalyzes both dehydrogenase and trans- 

 hydrogenase reactions with a number of pyridine nucleotide anal- 

 ogues as well as witli DPN and TPN. 



Enzyme Purification 



The placental enzyme was purified by Talalay, Hurlock, and Wil- 

 liams-Ashman ( 1958 ) . The purification procedure involved removal 

 of all particulate material by treatment with calcium chloride, heat 

 denaturation of inert protein, and fractionation with ammonium 

 sulfate. The enzyme is very unstable, and cstradiol-17/3 was added 



