292 R. R. PORTER [17 



minimum molecular weight found by end group assay of 14,000, The mono- 

 mer readily dimerizes on being brought to neutral or slightly alkaline reac- 

 tion in the absence of a reducing agent. The dimerization of human and 

 bovine serum albumin on addition of mercuric ion or a divalent organic 

 mercurial has been extensively studied at Harvard since the original obser- 

 vation of the phenomenon by Hughes (1950). It was shown that 70-80% 

 of the molecules possess a single SH group which is apparently unable to 

 interact with another SH group for steric reasons but which will cause 

 dimerization in the presence of Hg, It seems probable that the 12,000 frag- 

 ment which we have isolated possesses this single sulphydryl group but 

 owing to the removal of four-fifths of the original molecule the steric hind- 

 rance has been eliminated and these fragments can now dimerize freely. 

 Amino acid analysis shows big differences (Table 1) between the inhibitor 



Table 1 



AMINO ACID ANALYSIS OF INHIBITOR 

 AND OF SERUM ALBUMIN 



Results are expressed as g,/100 g. of protein 



and the whole molecule and it may be noted that there are 1 -4 molecules 



of cystine present suggesting that there is one disulphide bond and a free 



SH group. 

 The immunological activity of the fragment is lost if it is dissolved in 



■ * The amino acid content of serum albumin has been taken from Stein and Moore 

 (1949). 



t Tryptophan could not be detected spectroscopically with the Holiday photographic 

 method (Beaven and Holiday, 1952), 



