PHYSIOLOGICALLY ACTIVE COMPOUNDS 245 



mones of the posterior lobe of the pituitary (oxytocin and 

 vasopressin) are very similar in their amino acid composi- 

 tion," but even the small differences in the details of their 

 structure confer on each its own essential hormonal function 

 which it alone can carry out. 



In oxytocin the sequence is Cys. Tyr. lieu. Glu. Asp. Cys. 

 Pro. Leu. Gly. In vasopressin the sequence is Cys. Tyr. Phe. 

 Glu. Asp. Cys. Pro. Arg. Gly. 



Interesting results have been obtained by P. H. Bell and 

 R. G. Shepherd®* concerning the structure of the ^-adreno- 

 corticotrophic hormone (ACTH). This is a polypeptide with 

 a molecular weight of 5360 and the following arrangement 

 of amino acid residues: Ser. Tyr. Ser. Met. Glu. His. Phe. 

 Arg. Try. Gly. Lys. Pro. Val. Gly. Lys. Lys. Arg. Arg. Pro, 

 Val. Lys. Val. Tyr. Pro. Asp. Gly. Ala. Glu. Asp. Glu. Leu, 

 Ala. Glu. Ala. Phe. Pro. Leu. Glu. Phe. 



It is important that the biological activity of the hormone 

 only depends on the presence in the correct order of the 

 amino acids 1 - 24 starting from serine. The rest of the chain 

 is of no importance for its hormonal activity. 



Unfortunately we have not yet got the same information 

 concerning the sequences of amino acids in enzymes that we 

 have in hormones. In this case, however, the connection 

 between these sequences and the specific catalytic action of 

 the enzyme in question seems clearer. If the enzyme is to 

 hasten the transformation of the substance which acts as its 

 substrate it must first combine with that substance. 



For enzymes with two components w^hich have specific non- 

 protein (prosthetic) groups as well as proteins in their mole- 

 cules, it has long been established that the combination of 

 the enzyme with the substrate takes place through the pros- 

 thetic gi'oups.®^ 



For example, W. Langenbeck" showed in his model 

 experiments that the ability of carboxylase to catalyse the 

 decarboxylation of pyruvic acid is associated with the pres- 

 ence of an amino group in the molecule of the enzyme. 

 Simpler compounds containing this group, such as methyl- 

 amine, can also accelerate this reaction. The catalytic activity 

 of methylamine is, however, very slight, but can be made 

 many times greater by incorporating, in the molecule of 



