168 CONTROL MECHANISMS IN CELLULAR PROCESSES 



mones mediate the transfer of hydrogen between pyridine nucleo- 

 tides. These transhvdrogenase reactions are catalyzed by a soluble 

 enzyme isolated from human placenta. The concentrations of es- 

 tradiol- 17yS required to saturate this enzyme are very low and are 

 commensin-ate with those at which this hormone exerts its estrogenic 

 action. But the mechanism by which estrogenic steroids stimulate 

 these transhydrogenase reactions must be understood in precise 

 chemical terms before meaningful experimentation to assess their 

 wider physiological significance can be undertaken. 



The following topics are considered in this paper: experiments 

 with crude placental extracts; transhydrogenase reaction; reaction 

 mechanism; enzyme purification; binding and interaction of pyridine 

 nucleotides; reversibility; steroid specificitv; interconversion of hy- 

 droxy- and ketosteroids; stereospecificity of hydrogen transfer; in- 

 hibition experiments; model reactions with hepatic and bacterial 

 hydroxysteroid dehydrogenases; and biological implications. 



Experiments with Crude Placental Extracts 



Eight years ago, Hagerman and Villee ( 1953; Villee and Hager- 

 man, 1953) reported that the oxidative metabolism of glucose and 

 pyruvate by slices of human endometrium and placenta was ac- 

 celerated bv the direct addition of low concentrations (4 X lO"*' M) 

 of estradiol-17yS. Villee later extended these studies to cell-free ex- 

 tracts of human placenta from which particulate material was re- 

 moved bv ultracentrifugation (Villee and Hagerman, 1953; Villee, 

 1955; Gordon and Villee, 1955; Villee and Gordon, 1955). The re- 

 duction of DPN - ( but not of TPN ) by isocitrate catalyzed by these 

 extracts was stimulated profoundly by estradiol-17yS, and Villee sug- 

 gested that the site of action of the hormone was a soluble, DPN- 

 specific isocitric dehydrogenase. Hagerman and Villee ( 1957) found 

 that this "estrogen sensitive isocitric dehvdrogenase" was very un- 

 stable, and they were unable to purify the enzyme substantially. 

 They observed (Gordon and Villee, 1956) that estradiol-17/3 was 



- The following abbreviations are vised: DPN, diphosphopyridine nucleotide; 

 DI'NH, dihydrodiphosphopyridine nucleotide; TPN, triphosphopyridine nucleotide; 

 TPNH, dihydrotriphosphopyridine nucleotide; deaminoDPN, the hypoxanthine ana- 

 logue of diphosphopyridine nucleotide; acetylpyridineDPN, the 3-acetylpyridine 

 analogue of diphosphopyridine nucleotide; acetylpyridineTPN, the 3-acetylpyridine 

 analogue of triphosphopyridine nucleotide; pyriclincaldehydeDPN, the pyridine-3- 

 aldehyde analogue of diiDhosphopyridine nucleotide; thionicotinamideDPN, the thi- 

 onicotinamide analogue of diphosphopyridine nucleotide. 



