] 94 IMMUNO-CATALYSIS 



precipitates. Heidelberger and Kendall (1935) in their study on the 

 quantitative theory of the precipitin reaction found no difference, 

 within the range of pH 6.7 to 7.9, on the weight of antigen-antibody 

 precipitates. These investigators made no mention of the effect of lower 

 pH values. 



Pressman, et at. (1948) reported that the precipitation of p-azosuc- 

 cinanilate ovalbumin antiserum with homologous antigen, and that of 

 the protein antigen of antiserum specific to the p-azophenylarsonate 

 ion and the p-(p-azophenyl)-phenylarsonate ion is optimum at pH 

 values of 7.4 and 8.1 This range of pH is also optimal for the precipita- 

 tion of other azoprotein antigens with negatively charged haptenic 

 groups. 



Optimal pH for complete agglutination of sensitized cells has been 

 found to correspond with the iso-electric pH of the antibody globulin. 

 At the isoelectric pH of the antibody globulin, the largest amount of 

 sensitizing antibody is found to be absorbed by the cells (Coulter, 

 1920; DeKruif and Northrop, 1922-1923; Jolfe, 1935). 



It is apparent from these studies that the most favorable pH for 

 complete antigen-antibody combination is the region near to neu- 

 trality. It is desirable, however, that in an investigation of enzyme- 

 anti-enzyme reactions, this aspect of the question be carefully studied. 

 As an interesting illustration of the importance of pH on immune 

 reactions the findings of Northrop (1932) from a study of the serologi- 

 cal behavior of pepsin are described here. He immunized rabbits using 

 intraperitoneal injections with either active or inactive pepsin prepara- 

 tions. The immune serum against active pepsin gave a precipitate with 

 active pepsin in a dilution of about 1 : 2000, and with inactivated 

 pepsin in a dilution of 1:16. The serum against alkali inactivated 

 pepsin gave a precipitate with both the inactivated and active pepsin 

 in a dilution of about 1 :8. 



The inhibition of the activity of pepsin (tested with a casein solu- 

 tion) by either of the immune sera was about the same and not very 

 much greater than the inhibition by normal serum. The inhibitory 

 effect, measured by the rate of hydrolysis of gelatin with a small 

 amount of pepsin to which increasing amounts of the various sera 

 were added, was about 40 per cent with serum against active pepsin 

 in a dilution of 1:4, and about 20 per cent with denatured pepsin 

 serum, and still less with normal serum. The weak inhibitory effect 



