318 CHOH HAO LI [18 



the amino acids which are released by carboxypeptidase from the reduced 

 form of either beef or sheep prolactin.^' 



H-Thr. Pro. Vol. Thr. Pro.tmi^LmmÊÊÊmiÊJLmm 



Fig. 8. Proposed gross structure for the prolactin molecule. (S — 8)2 indicates schematic- 

 ally the existence of two loops containing disulfide bridges. This model is somewhat 

 speculative and structures other than that shown here are possible. 



IV. GROWTH HORMONES (SOMATOTROPINS) 



The relation of the pituitary gland to growth was first given experimental 

 demonstration by Gushing and his collaborators^* in their classical experi- 

 ments on the hypophysectomy of dogs in 1910, and by 1930 investigations 

 with various species, including fish, amphibia, reptiles, birds and mammals, 

 had left little doubt that the anterior pituitary contains a hormone which 

 initiates resumption of growth in hypophysectomized animals. During the 

 period between 1944 and 1948, methods for the isolation of growth hor- 

 mone in a highly purified form from the anterior lobes of bovine pituitaries 

 were described by two groups of investigators.®^®* During the past few 

 years, the many attempts®' •®^-®^''° that have been made to purify further 

 the products isolated by the pubUshed procedures®^®® have not met with 

 success. The failure of these attempts has lent support to the belief that the 

 protein is the hormone.^"-'^ 



Bovine Growth Hormone. The bovine somatotropin is a protein with a 

 molecular weight of 45,000 and an isoelectric point of pH 6-85; the protein 

 nitrogen can be accounted for completely by its amino acid and amide 

 content. ^° The hormone protein contains a total of 396 amino acids; structur- 

 ally, it is a branched polypeptide chain with two TV-terminal residues (phenyla- 

 lanine and alanine) and only one C-terminal residue (phenylalanine),* as 

 shown in Fig. 9. 



When the bovine growth hormone from which two residues of phenyla- 

 lanine had been removed by the action of carboxypeptidase was assayed 

 for biological activity, the C-terminal phenylalanine was found not to be 

 essential for the biological function of the hormone.'^ Ghymotryptic diges- 

 tion, performed at a temperature of 25°, in a buffer of pH 9-5 and with an 



* Recent work with H. Papkoff has shown that growth hormone isolated from whole 

 sheep pituitary glands by the same procedure as that described for beef glands has a 

 similar structure to that of the bovine somatotropin, with two A^-terminal residues (phenyla- 

 lanine and alanine) and one C-terminal residue (phenylalanine). Its molecular weight is 

 47,800 and its isoelectric point is at pH 6-8. 



