174 CONTROL MECHANISMS IN CELLULAR PROCESSES 



tectable inhibition of this dehydrogenase reaction occurred when the 

 DPN/TPN ratio was 10\ Similarly, the oxidation of estradiol-17;Q 

 by acetylpyridine DPN was powerfully inhibited by TPN. But the 

 reduction of TPN bv the steroid was unaffected by DPN. If it is 

 assumed that the different pyridine nucleotides compete for the same 

 binding site ( s ) on the enzyme, then these inhibitions of various de- 

 hydrogenase reactions are in accord with the relative affinities of the 

 nucleotides. 



The affinities for different pyridine nucleotides in the dehydro- 

 genase reaction appeared to give some insight into the nucleotide 

 interactions in various transhydrogenase systems. The rate of the 

 TPNH-DPN exchange was very dependent upon the relative con- 

 centrations of DPN and TPNH. At each level of DPN, there is an 

 optimum concentration of TPNH. At pH 7.4, the optimum DPN/ 

 TPNH ratio is in the region of 50-200 (Talalay, Hurlock, and Wil- 

 liams-Ashman, 1958 ) . The transfer of hydrogen from TPNH to DPN 

 is inhibited at high concentrations of TPNH. Transhydrogenation 

 between TPNH or DPNH and their corresponding 3-acetylpyridine 

 analogues is exquisitely sensitive to the relative concentrations of 

 nucleotides in the reaction mixture. Thus, the DPNH-acetylpyri- 

 dineDPN, the DPNH-acetylpyridineTPN, and the TPNH-acetylpyri- 

 dineTPN exchange reactions proceed in the presence of relatively 

 high levels of the donor nucleotides, whereas the transfer of hydro- 

 gen from TPNH to acetvlpyridineDPN is detectable only if the con- 

 centration of TPNH is kept very low. Again, transhydrogenation 

 between DPNH and either acetylpyridineDPN or pyridinealdehyde- 

 DPN is inhibited by very low concentrations of TPN or TPNH. 



Reversibility 



The oxidation of estradiol-lT^S to estrone with DPN or TPN as 

 hydrogen acceptor is readily reversible at pH 7.4. Langer and Engel 

 (1958) reported the equilibrium constant K=(DPNH) (H+) 

 ( Estrone )/( DPN +) (Estradiol-17y8) to be 1.8 X 10"^ M at 25° (cf. 

 Talalay, 1957a). Since the oxidation reduction potentials of the 

 DPNH-DPN and TPNH-TPN couples are the same, it might be ex- 

 pected that the estradiol-mediated transfer of hydrogen from TPNH 

 to DPN would be readily reversible. But Talalay, Hurlock, and Wil- 

 liams-Ashman ( 1958 ) found that the placental enzyme did not cata- 

 lyze the transfer of hydrogen from low levels of DPNH ( generated 



