TRANSHYDROGENASE REACTIONS AND ESTROGENS 177 



of hydrogen to side 11 (yS) (ct. Levy and Vennesland, 1957) of the 

 nicotinamide moieties of TPN and DPN. Hydrogen transfer from 

 DPNH to acetvlp\'ridineDPN mediated bv estradiol-17/3 hkewise 

 involves the abstraction of hydrogen from side II of the DPNH. 



Inhibition Experiments 



In crude placental extracts, Hollander, Hollander, and Brown 

 ( 1959a ) observed that the dehydrogenase and transhydrogenase 

 functions of crude placental extracts were inhibited to about the 

 same extent upon storage at 3° at various hvdrogen ion concentra- 

 tions. Hagerman and Villee ( 1959 ) reported that heating a partially 

 purified placental preparation to 56° for 2 hours inactivated the 

 DPN-dehydrogenase activity to a greater extent than that of the 

 TPN-dehydrogenase; the TPNH-DPN transhydrogenase reaction 

 was much less inhibited. But Talala)' and Williams-Ashman ( 1960) 

 were unable to demonstrate differences in the heat stability of the 

 estradiol-DPN dehvdrogenase reaction and of transhvdrogenation 

 between DPNH and acetylpvridincDPN. 



However, differential inhibition of various dehydrogenase and 

 transhydrogenase functions can be demonstrated under some experi- 

 mental conditions. For example, the concentration of adenosine-2'- 

 monophosphate and of 2\ 5'-adenosine diphosphate required to in- 

 hibit the oxidation of estradiol-17/3 bv DPN is considerably greater 

 than that needed to depress the TPNH-DPN transhydrogenase re- 

 action (Hollander, Hollander, and Brown, 1959c). Converselv, both 

 p-chloromercuriphenyl sulfonic acid and thyroxine inhibit the estra- 

 diol-DPN dehydrogenase reaction at lower concentrations than are 

 required to inhibit either the TPNH-DPN or the DPNH-acetylpyri- 

 dineDPN exchanges (Hagerman and Villee, 1959; Villee, Hagerman, 

 and Joel, 1960) . But in these inhibition experiments, dehydrogenase 

 and transhydrogenase activities were not measured under identical 

 experimental conditions, especially in relation to pyridine nucleotide 

 concentration. Thus, they do not provide unequivocal evidence for 

 separate enzymes catahzing translivdrogenation and dehydrogena- 

 tion, as Hollander, Hollander, and Brown (1959c) point out. 



It is noteworthy that the temperature coefficients for the reduc- 

 tion of different pyridine nucleotides by estradiol-17/3 vary widely 

 with the purified placental enzyme. Large differences in the relative 

 as well as the absolute rates of various dehydrogenase and transhy- 



