294 R. R. PORTER [17 



the inhibitor contains the whole surface of the combining centre which 

 binds more firmly to the antibody site than the smaller haptenic groups. 

 The importance of configuration in the former suggests that a relatively 

 large surface area is involved. With a small haptenic group the secondary 

 valence forces would be less and binding correspondingly less firm. 



The dimerization of the inhibitor had no influence on its immunological 

 behaviour showing that the surface adjacent to the sulphydryl group, which 

 would be blocked in the dimer, could not be a part of the combining site. 

 As the work on the dimerization of the whole molecule suggests that the 

 sulphydryl group is sunk into the surface and is not readily accessible, it 

 is perhaps not surprising that it is not part of a combining site. Unless an 

 antigen is partly broken down prior to its participation in stimulating anti- 

 body formation, it would be expected that only the surface of the molecule 

 determines the complementary character of the antibody combining site. 

 It should also be noted that the part of the inhibitor surface adjacent to 

 the sulphydryl group could play an essential part in determining the con- 

 figuration of the combining centre, though not itself part of it. 



The inhibitor will cause anaphylactic shock when injected into a guinea 

 pig positively immunized with rabbit anti-bovine serum albumin serum and 

 at about the same level as that required to cause shock by injection of the 

 whole antigen. Landsteiner and Van der Scheer (1933) showed that azo dye 

 haptenes would cause anaphylactic shock in sensitized guinea pigs, if the 

 dyes used had two haptenic groups. This type of haptene will also cause 

 precipitation of antisera, but with the inhibitor molecule there does not 

 appear to be any correlation between precipitating power and ability to 

 cause anaphylactic shock. 



SUMMARY 



A 12,000 mol. wt. fragment of bovine serum albumin has been isolated from a 

 chymotryptic digest. 



This fragment retained the power to combine with the specific antibody 

 as demonstrated by inhibition and precipitation studies and also by its 

 power to provoke anaphylactic shock in sensitized guinea pigs. 



The fragment readily dimerizes at neutral or alkaline reaction and evidence 

 is presented which suggests that this dimerization is caused by interaction 

 of SH groups, probably the same groups which lead to the formation of 

 a Hg dimer mercaptalbumin with the original molecule. 



REFERENCES 



BEAVEN, G. H. and HOLIDAY, E, R. (1952), Advanc. Protein Chem., 7, 319. 

 HARRIS, J. I. and KNIGHT, c. A. (1952), Nature, London, 170, 613. 

 HARRIS, J. I. and LI, c. h. (1952), /. Am. chem. Soc, 74, 2945. 

 HARRIS, J. I., LI, c. H., CONDLIFFE P. G. and PON, N. G. (1954), /. biol. Chem. 

 209, 133. 



