262 
CHEMISTRY: E. J. COHN 
Proc. N. a. S. 
isoelectric points, and the decomposition of their compounds with bases. 
If the isoelectric points of these globulins had not been acid, but had oc- 
curred at an alkaline reaction, their acid compounds would have been de- 
composed by bases, and as a result they would probably have been rendered 
less soluble by bases rather than by acids. 
Dissociation of Tuberin in Acid Solution. — In the presence of hydrogen 
ion concentrations greater than the isoelectric point, tuberin ionized as a 
base and dissolved as an acid compound. The effect of sodium chloride 
in precipitating tuberin from such solutions is perhaps less clear. Hardy 
studied the identical effect upon serum globulin. He believed that pro- 
teins formed addition products with neutral salts through their amino 
groups, and that under these circumstances neutral salts displaced acid 
from combination with protein. If that were so the acidity of such sys- 
tems would increase. We have seen that in the case of tuberin quite the 
contrary occurs. Tuberin chloride binds more rather than less acid in 
the presence of sodium chloride. For the present, therefore, it is simpler 
merely to assume that tuberin chloride is little soluble when little dis- 
sociated. 
Classification of Tuberin as a Globulin. — The classification of tuberin as 
a globulin depends, as our observations indicate, upon the solubility in 
salt solutions of the compounds that tuberin forms when ionized as an acid, 
and upon the insolubility of the products of the hydrolytic dissociation 
of these same compounds. It would appear from Osborne's observations 
upon the behavior of many other globulins toward small amounts of acid, 
that they too have acid isoelectric points, and are, therefore, ionized as 
acids in neutral salt solutions. If the isoelectric points of these proteins 
had been alkaline, their solubility in salt solution would have been de- 
creased by bases rather than by acids. But whether the relations that 
have been described are characteristic of all globulins, or merely of tuberin, 
it appears from them how intimately the classification of proteins accord- 
ing to solubility involves their isoelectric points. 
1 am greatly indebted to the Director of the Harriman Research Labora- 
tory, the Roosevelt Hospital, New York, for courteously extending to me 
the facilities of his laboratory after the research that I had been conducting 
there for the Surgeon General's Office was concluded. This enabled me 
to consider these more theoretical aspects of the earlier problem. 
^ Recommendations of the Committee on Protein Nomenclature, Amer. J. Physiol., 
21, 1908 (27). 
2 Osborne, T. B. and Campbell, G. F., /. Amer. Chem. Soc, 18, 1896 (575). 
3 Cohn, E. J., Gross, J. and Johnson, O. C, /. Gen. Physiol, 1919, II, (145). 
4 Osborne, T. B. and Campbell, G. F., /. Amer. Chem. Soc, 19, 1897 (482-487). 
6 Osborne, T. B., Ibid., 24, (28-39). 
6 Hardy, W. B., Proc. Roy. Soc, London (B), 79, 1907 (413-426). 
7 Hardy, W. B., /. Physiol., 33, 1905-6 (251-337). 
8 Rona, P. and Michaelis, h., Biochem. Zs., 28, 1910 (193-199). 
