Vol. 6, 1920 
CHEMISTRY: E. J. COHN 
259 
combined with fewer of the electrolytes that existed in potato juice or were 
formed by the addition of acid to it. For this reason the flow of current 
during cataphoresis was greatest at the isoelectric point (3, p. 155). At 
hydrogen ion concentrations greater than the isoelectric point, where 
tuberin migrated to the cathode, and bore, therefore, a positive charge, 
it was evidently ionized as base and combined with acid. 
Solubility of Acid and oj Basic Tuberin Compounds. — The coincidence 
of the hydrogen ion concentration at which sodium chloride has least 
effect upon the solubility of tuberin with the isoelectric point allows of 
but one interpretation; sodium chloride affects tuberin differently as the 
migration of the protein varies. At hydrogen ion concentrations greater 
than 10"^ Ny where tuberin was ionized as base and combined with acid, 
sodium chloride decreased its solubility. When tuberin was least ionized, 
at its isoelectric point, sodium chloride had least effect upon its solubility. 
Expecially when tuberin was ionized as acid did sodium chloride increase 
its solubility in the way that has led to its being classified as a globulin. 
Solubility of Other Globulins. — This conception of the behavior of a globu- 
lin is not new. It was expounded by W. B. Hardy in his Croonian Lec- 
ture of 1905^ on the basis of his studies of serum globulin.'^ Excepting 
in one point, which will presently be considered, his observations and con- 
clusions agree with ours on tuberin. Neutral salts only increased the 
solubility of serum globulin at reactions more nearly neutral than its 
isoelectric point. At acidities greater than the isoelectric point, neutral 
salts precipitated serum globulin, as they did tuberin. 
The earlier observations that have been mentioned indicate (1) that 
the solubility of many other globulins in salt solutions is decreased by small 
amounts of acid, and (2) that many globulins are precipitated from acid 
solution by small amounts of neutral salt. Up to the present the isoelectric 
points of most of these proteins have not been determined. * The way in 
which they are ionized is, however, suggested by these criteria. This 
appears from a consideration of the effect of sodium chloride upon the dis- 
sociation of the compounds that tuberin forms at greater and at lesser 
acidities than correspond to its isoelectric point. 
II. THE EFFECT OF SODIUM CHLORIDE UPON THE ACID COMBINING CAPACITY 
OF TUBERIN 
The chemical analysis of the effect of sodium chloride upon the ioniza- 
tion of the different tuberin compounds, whose solubility it differently 
affects, is carried one step further by the measurements of hydrogen ion 
concentration. The hydrogen ion concentration of a system containing 
protein and acid or protein and base is, as a first approximation, defined 
by the ratio of protein to acid, or protein to base,^'^^'^^ and depends merely 
* Rona and Michaelis^ give the isoelectric point of serum globulin as 0.36 X 10~^ 
and of edestin as 1.3 X 10~^ These values depend in part, however, upon the points 
of maximum precipitation of the proteins. 
