1908. | | Diphtheria Antitoxin. 407 
after four and nine days’ tryptic digestion is 15 per cent. The precipitation 
limits of the digested protein only are altered. 
The first result indicates that the slow action of trypsin on the proteins of 
serum is due to the resistance of this type of protein to tryptic digestion 
rather than to the presence of an:antibody to trypsin in the serum. The 
second result emphasises the need for quantitative experiment when 
precipitating digested mixture of protein with ammonium sulphate, more 
especially if conclusions as to the antitoxic value of the precipitate are 
required, 
The rate of digestion of the protein as given by the above figures is 
approximately the rate at which the antitoxin is destroyed by trypsin. 
Conclusion. 
The results of all the above experiments on diphtheria antitoxin may be 
briefly stated :— 
(a) Whenever the main protein of serum is precipitated, diphtheria anti- 
toxin is precipitated. 
(6) Whenever the main protein of serum is destroyed or coagulated, 
diphtheria antitoxin is destroyed. 
These results have only one logical explanation—that diphtheria antitoxin 
is a protein. | 
There is a possibility that diphtheria antitoxin is a comparatively simple 
body, and is attached to the protein only by virtue of some chemical affinity. 
But in the above experimental work a large number of methods of analysis 
have been used, and it is reasonable to suppose that if the hypothesis above 
were correct, some indication would have been obtained of a splitting of the 
antitoxin from its accompanying protein. No such indication has been 
obtained. If we assume, then, that the diphtheria antitoxin is a protein, what 
hope is there of isolating it in a pure condition? Serum doubtless contains 
a very large number of proteins which possess different physiological proper- 
ties. But a living cell is required to appreciate these physiological differences. 
In laboratory work the only available forces are those of a physical and 
chemical nature. With these forces it is possible to differentiate the proteids 
of horse serum into three groups :— 
(a) Globulins comprising about 3 per cent. of the total protein. 
(6) Albumins which cannot be crystallised. These compose about 85 per 
cent. of the total proteins. 
(c) Albumins which may be crystallised. These compose about 12 per cent. 
of the total protein. 
