d -^ ■ ^' 



i: '* 



18] STRUCTURE OF PITUITARY HORMONES 315 



This appears to be consistent with the earlier findings of Fleischer''^ that in 

 alcoholic solution the bovine hormone is more soluble than the ovine. 



Table 6 presents a summary of the known physical and chemical pro- 

 perties of prolactin from beef and sheep pituitary glands. The tyrosine con- 

 tent as determined by the spectrophotometric method is definitely higher 



Table 6 



SOME PHYSICAL AND CHEMICAL PROPERTIES 



OF PROLACTIN FROM SHEEP AND BEEF 



PITUITARY GLANDS 



for the beef hormone than for the sheep hormone. Both hormones have 

 a molecular weight of 26,000 and an isoelectric point at pH 5-73; they both 

 consist of a single peptide chain with 211 amino acid residues^^ and the 

 sequence Thr.Pro.Val.Thr.Pro . . . at the TV-terminus.^" Studies' in which 

 prolactic preparations of both species were subjected to reaction with carb- 

 oxypeptidase and to hydrazinolysis, have given no indication that either 

 hormone possesses a C-terminal residue at all. Prolactin that has undergone 

 oxidation with performic acid has been found to behave as a single com- 

 ponent in electrophoresis, sedimentation, and chromatography, as well as 

 according to the results of A^-terminal residue analysis. ^^ From these data, 

 as well as from the absence of evidence of any C-terminal residue, it was 

 postulated tentatively that the prolactin molecule has a cyclic configuration 

 at the C-terminus.' This tentative structure has now been verified by the 



♦ Determined in 0-1 M-NaHCOa solutions. 



t Estimated in acetate buflfers of 05 ionic strength. 



