334 J. lEUAN HARRIS [18 



corresponding positions in the corticotropin molecule. The structural simi- 

 larity between j3-MSH and corticotropin is of particular interest since it 

 provides a chemical basis for the observation (Bell, 1954; Dixon, 1956a) 

 that pure corticotropin preparations possess intrinsic melanocyte stimulating 

 activity in addition to their primary adrenocorticotropic action. 



<■ u >f<- 



« Ti >|< Tj >J 



I 234 S 6^7 es lO II Xi: 



Asp.GluGly.Pro. Tyr. Lys. Met.Glu.His.Phe. Arg. Tr,. _., . . _. . ._._,„.. ,^. 



T Te Che -Wk- Chs - - 



Ch, »U- — Ch, >J<! — Ch, 



Fig. 1. Amino-acid sequence of /3-meIanocyte-stimulating hormone (pig /3-MSH); 

 arrows indicate points of cleavage by trypsin (T) and chymotrypsin (C). 



Geschwind, Li and Barnafi (1956, 1957a) have reported similar conclu- 

 sions concerning the melanocyte stimulating peptide which they isolated 

 from pig posterior pituitary extracts, showing that it was identical in all 

 respects with the material isolated by Porath et al. (1955) and characterized 

 by Roos (1956). Geschwind, Li and Barnafi (1957Z>) have also reported the 

 isolation and structure of a jS-type melanocyte-stimulating hormone from 

 ox pituitaries; j8-MSH (ox) was found to differ from j8-MSH (pig) only in 

 position 2 (Fig. 1), where the glutamic acid residue in pig j8-MSH is replaced 

 by a serine residue in the ox hormone. 



THE a-MELANOCYTE-STIMULATING HORMONE 



(a-MSH) 



The structure of a-MSH has been determined by Harris and Lerner (1957), 

 using material prepared by Dr Teh Lee according to the procedure described 

 by Lee and Lerner (1956). Although it did not contain either a free TV-terminal 

 or C-terminal group, the amino acid sequence of a-MSH could be deduced 

 from the structure of peptide fragments derived from it by digestion with 

 trypsin and chymotrypsin. The results are summarized in Fig. 2. 



R-Ser. Tyr. 



1 



Ser. Met. Glu. His. Ph« Arg. Try. Gly. Lys. Pro. Vol.- NH 



t 



-Ch4->T< Chs- - — 



-> 



<«• -Ch, - >|< Ch2 ->l< Chj— ___ -> 



Fig. 2. Amino-acid sequence of a-melanocyte-stimulating hormone (pig a-MSH); 

 arrows indicate points of cleavage by trypsin (T) and chymotrypsin (C). 



a-MSH is shown to consist of thirteen amino acid residues in a single 

 peptide chain, and the amino acid sequence is identical with the A^-terminal 

 tridecapeptide sequence in corticotropin. However, it differs from the re- 

 lated portion of the corticotropin molecule in that the A^-terminal residue. 



