DOUGLAS M. SURGENOR 



of a chemical messenger released into and transported by the 

 blood stream. However, experimental proof of the existence of 

 hormones in the blood has been very slow in accumulating. 

 The difficulty has been partly one of technique, for even under 

 optimal conditions the plasma level of a given hormone is 

 extremely low and is variable with time. Assays which have 

 been developed and used during isolation and purification of 

 hormones from endocrine tissue have not generally been suffi- 

 ciently sensitive to determine plasma levels. This difficulty of 

 assay was circumvented in the case of the thyroid hormone, by 

 direct analysis for protein-bound iodine. It has been found (10) 

 that the iodine in plasma is concentrated in a protein fraction 

 (Fraction IV-4) under conditions where free thyroxin (MW = 

 777) should have been soluble, suggesting that the physical 

 chemical state of the hormone in the plasma is different from 

 that of the free hormone. The plasma protein has not yet been 

 purified, however, and its possible relationship to the glandular 

 tissue protein, thyroglobulin, remains to be established. During 

 World War II, in collaboration with Hisaw, activities ascribable 

 to luteinizing, follicle-stimulating, and thyrotropic hormones 

 were also observed in plasma fractions (10). Later, the estro- 

 genic hormone estriol was identified as a specific constituent 

 of the /3-lipoprotein by Roberts and Szego, who also made 

 some interesting observations on its state of association with 

 the huge lipoprotein molecule (30). Quite recently in our 

 laboratory, insulin-like activity has been reproducibly demon- 

 strated in fractions from small pools of human plasma and, by 

 using selected donors, in whom elevated amounts of certain 

 hormones are known to occur, gonadotropic hormone activity 

 has been partially concentrated from post-menopausal plasma, 

 and growth hormone activity has been demonstrated, although 

 less easily, in the plasma from acromegalic donors. These and 

 other findings strongly support the original concept of trans- 

 mission of hormonal activity by the blood stream. Many 

 important questions remain, however, for an understanding of 

 the individual mechanisms involved. 



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