178 CONTROL MECHANISMS IN CELLULAR PROCESSES 



drogenase activities are found if the reactions are studied at different 

 temperatures (Talalay and Williams-Ashman, 1959). 



Model Reactions with Hepatic and Bacterial 

 H/droxysteroid Dehydrogenases 



It would apear at first sight that hydrogen transfer from one form 

 of pyridine nucleotide to another should be catalyzed by any dehy- 

 drogenase which reacts with the requisite nucleotides in the pres- 

 ence of its substrate. However, transhydrogenation between TPNH 

 and DPN is not catalyzed by a yS-hydroxyacyl-CoA dehydrogenase 

 which is specific for both DPN and TPN (Stern, 1958). Moreover, 

 glutamic (cf. Colowick, Kaplan, Neufeld, and Ciotti, 1952) and 

 lactic dehydrogenases do not promote the TPNH-DPN exchange 

 unless the concentration of enzyme, substrate, and pyridine nucle- 

 otide is extraordinarily high ( Holzer and Schneider, 1958 ) . The liver 

 of the guinea pig contains two testosterone dehydrogenases ( Kocha- 

 kian, 1959 ) . One of these enzymes is specific for DPN and is bound 

 to cytoplasmic particles, while the other is a soluble dehydrogenase 

 which reacts only with TPN. According to Villee, Hagerman, and 

 Joel ( 1960), a mixture of these two testosterone dehydrogenases does 

 not catalyze the transfer of hydrogen from TPNH to DPN in the 

 presence of testosterone. These authors also state that the TPNH- 

 DPN transhydrogenase reaction is not catalyzed by a mixture of 

 their DPN- and TPN-specific placental estradiol dehydrogenases 

 upon the addition of catalytic quantities of estradiol-17/3. 



However, steroid-dependent hydrogen transfers between pyridine 

 nucleotides have been shown unequivocally with a number of hy- 

 droxysteroid dehydrogenases. Hurlock and Talalay (1958) found 

 that the soluble 3a-hydroxysteroid dehydrogenase of rat liver cata- 

 lyzes an androsterone-dependent TPNH-DPN exchange according 

 to the following equations: 



Androstane-3, 17-dione + TPNH + H+ ^ Androsterone + TPN+ 



Androsterone + DPN+ «=> Androstane-3, 17-dione + DPNH + H+ 



TPNH + DPN+ ^ TPN+ + DPNH 



This was confirmed by Koide, Chen, and Freeman (1960). This 

 3a-hydroxysteroid dehydrogenase also catalyzes an androsterone- 



