CLAKA M. SZKGO l6i 



live of a mechanism by which 'acLivated' transport forms of the steroid as 

 specific lipoprotein complexes are synthesized in vivo. 



These investigations have been extended to the resolution of the above 

 incubation media by paper electrophoretic and cold ethanol fractionation 

 techniques (37, 39). The results demonstrate that the in vilro association of the 

 steroidal moiety with the serum proteins appears to occur selectively with 

 albumin in the presence of rat liver tissue. The data substantiate the conclusion 

 that this association is a reflection of estrogen-protein binding, rather than 

 simple coprecipitation. Whether or not endogenous estrogen is present in rat 

 serum bound to albumin is unknown. It should be noted, however, that in 

 animals other than man, albumin may assume a lipide-transporting role in 

 the absence of those jS-lipoproteins peculiar to human plasma (cf. 39). 



These methods have been useful in the analysis of the antagonistic influence 

 exhibited by the ii-oxy, 17-hydroxy adrenocortical steroids upon the metabolic 

 responses of the uterus to estrogenic stimulation. At the time these observations 

 were reported (cf. 24, 26) the suggestion was advanced that the phenomenon 

 might be due to competition by the two classes of steroid for active sites on 

 protein molecules which have functions essential for steroid activity — either 

 in terms of transport complexes apparently formed in the liver, or at the locus 

 of the target organ itself. This suggestion was predicated upon the following 

 lines of evidence: o) the specific structural requirements for antagonistic 

 effects, b) the 'titrability' of these effects with increasing dosage of the appro- 

 priate antagonist, c) the observation that the antagonists were found to have 

 no effect per se on the metabolic processes which were stimulated by the estro- 

 gens, even though they were capable of inhibiting estrogenic stimulation of 

 these processes, and (/) the demonstrated affinity of certain of the steroid hor- 

 mones for specific proteins. 



This hypothesis appears to gain support from recent studies of the influence 

 of increasing levels of unlabeled Cortisol upon the liver-catalyzed association 

 of isotopic estrone with rat serum albumin (38, 39). It was observed that 625 

 micrograms of Cortisol (compound F) significantly inhibited the degree of 

 estroprotein formation seen in the control containing isotopic estrone alone, and 

 that doubling the Cortisol level virtually abolished the process. The large diver- 

 gence in order of magnitude of the adrenocortical vs. estrogenic steroids is in 

 line with the in vivo observations, and suggests that the relative affinities of 

 the two groups of hormones for the protein is vastly in favor of the acidic estro- 

 gens, which appear by all criteria to be the most firmly bound of all the steroids 

 thus far investigated (cf. 26). This is in harmony with the extremely low order 

 of magnitude of their physiological dosage requirements, and if protein binding, 

 as has been postulated, is a necessary preliminary- to steroid hormone action, 

 a rational explanation for the greater activity of estrogenic substances would 

 appear at hand. 



