The Immunological Specificity of the Euglobu- Lin and Pseudoglobulin Fractions of Horse and Human Serum

Serum is generally considered to contain at least two proteins, albumin and globulin, differing both in their isoelectric points, (literature summarized by Freundlich) and molecular weights (Svedberg and Sj6gren), and readily separable by half-saturation with ammonium sulfate, which precipitates the globulin. There has been considerable investigation tending to show that the globulin fraction itself is not homogeneous, but consists of two distinct entities. Thus we find many early investigators dividing the total globulin into water-soluble and water-insoluble fractions, which they called pseudoglobulin and euglobulin respectively, whereas others have applied the term euglobulin to that fraction of the serum protein precipitated by 33 per cent saturation with ammonium sulfate, and the term pseudoglobulin to the fraction precipitated between 33 and 50 per cent saturation (literature summarized by Kato). Such differences in solubility do not, however, necessarily mean that the fractions are actually different proteins. The fact that only part of the serum globulin is precipitated on 33 per cent saturation with ammonium sulfate hardly suffices to distinguish two fractions, as even proteins known to be homogeneous can be partially precipitated by arbitrarily chosen precipitating agents. With respect to water-solubility as a distinguishing criterion between protein fractions, there are reports that the water-soluble fraction undergoes a progressive loss in solubility as it ages, presumably going over to euglobulin (Chick), and that simple dialysis does not suffice to effect complete separation, which is achieved only by electrodialysis (Ruppel, Stern). Finally, Svedberg and SjSgren report that globulin