654 



PHYSIOLOGY OF GONADS 



between serine and glycine plus an active 

 one-carbon unit. They found that serine 

 aldolase activity, measured in homog- 

 enates of rat uteri, increased 18 hours after 

 pretreatment in vivo with estradiol. It 

 seemed that the estrogen-induced increase 

 in the activity of this enzyme might explain 

 at least part of the increased rate of one- 

 carbon metabolism following estrogen in- 

 jection. They found, however, that incuba- 

 tion of uterine segments in tissue culture 

 medium (Eagle, 1955) for 18 hours pro- 

 duced a marked increase in both the activity 

 of serine aldolase and the incorporation of 

 glycine-2-C^'* into protein. The addition of 

 estradiol to Eagle's medium did not produce 

 a greater increase than the control to 

 which no estradiol was added. Uterine seg- 

 ments taken from rats pretreated with estra- 

 diol for 18 hours, with their glycine-incor- 

 porating system activated by hormonal 

 stimulation, showed very little further 

 stimulation on being incubated in Eagle's 

 medium for 18 hours. With a shorter period 

 of i^retreatment with estradiol, greater stim- 

 ulation occurred on subsequent incubation 

 in tissue culture fluid. These experiments 

 suggest that the hormone and the incuba- 

 tion in tissue culture medium are affecting 

 the same process, one which has a limited 

 capacity to respond. When comparable ex- 

 periments were performed with other 

 labeled amino acids as substrates, similar 

 results were obtained. 



Mueller's work gave evidence that a con- 

 siderable number of enzyme systems in the 

 uterus are accelerated by the administration 

 of estradiol — not only the enzymes for the 

 incorporation of serine, glycine, and formate 

 into adenine and guanine, but also the en- 

 zymes involved in the synthesis of fatty 

 acids and cholesterol and indejX'ndent en- 

 zymes for the activation of amino acids by 

 the formation of adenosine monoiihosphate 

 (AMP) derivatives. The initial step in 

 protein synthesis has been shown to be the 

 activation of the carboxyl grou]) of the 

 amino acid with transfer of energy from 

 ATP, the formation of AMP -"amino 

 acid, and the release of jiyrophosphate 

 (Hoagland, Keller and Zamecnick, 1956). 

 This reversible step was studied with ho- 

 mogenates of uterine tissue, P^--labeled 

 ]n'rni)liosi)liate, and a variety of amino 



acids (Mueller, Herranen and Jervell, 

 1958). Seven of the amino acids tested, 

 leucine, tryptophan, valine, tryosine, methi- 

 onine, glycine, and isoleucine, stimulated the 

 exchange of P^^ between pyrophosphate and 

 ATP. Pretreatment of the uteri by estradiol 

 injected in vivo increased the activity 

 of these three enzymes. The activating 

 effect of mixtures of these amino acids 

 was the sum of their individual effects, 

 from which it was inferred that a specific 

 enzyme is involved in the activation of 

 each amino acid. Since estrogen stimu- 

 lated the exchange reaction with each of 

 these seven amino acids, Mueller con- 

 cluded that the hormone must affect the 

 amount of each of the amino acid-activat- 

 ing enzvmes in the soluble fraction of the 

 cell. 



Mueller (1957) postulated that estrogens 

 increase the rate of many enzyme systems 

 both by activating preformed enzyme mole- 

 cules and by increasing the rate of de novo 

 synthesis of enzyme molecules, possibly by 

 removing membranous barriers covering the 

 templates for enzyme synthesis. To explain 

 why estrogens affect these enzymes in the 

 target organs, but not comparable enzymes 

 in other tissues, one would have to assume 

 that embryonic differentiation results in 

 the formation of enzymes in different tissues 

 which, although catalyzing the same re- 

 action, have different properties such as 

 their responsiveness to hormonal stimula- 

 tion. 



As an alternative hypothesis, estrogen 

 might affect some reaction which provides 

 a substance required for all of these en- 

 zyme reactions. The carboxyl group of 

 amino acids must be activated by ATP be- 

 fore the amino acid can be incorporated 

 into proteins; the synthesis of both purines 

 and pyrimidines requires ATP for the 

 activation of the carboxyl group of certain 

 precursors and for several other steps; the 

 synthesis of cholesterol requires ATP for 

 the conversion of mevalonic acid to 

 squalene; and the synthesis of fatty acids 

 is also an energy-requiring process. Thus if 

 (>strogens acted in some way to increase the 

 amount of biologically useful energy, in 

 the form of ATP or of energy-rich thioesters 

 such as acetyl coenzyme A, it would increase 

 the rate of syntliesis of all of these compo- 



