Globulins

GLOBULINS, a group of animal and plant proteins possessing a number of common properties, chiefly of a physicochemical character, on the basis of which they are distinguished into a separate category. They are insoluble in pure water, but dissolve in the presence of neutral salts, in dilute alkalis and acids. They precipitate from solution upon a decrease in salt content (dilution, dialysis) or upon an excess of acid (even CO₂). They are completely precipitated by saturating their solution with MgS0₄ at 30° and by half-saturation with ammonium sulfate. On the basis of their precipitation by (NH₄)₂S0₄, globulins can be subdivided into further fractions: euglobulins, which precipitate already at ¹/₃ saturation, and pseudoglobulins at half-saturation. However, in these fractions, and even more so in the globulins themselves, there are by no means definite chemical individuals. The matter essentially comes down to a difference in the size of the particles, in the degree of hydration, etc. Some chemical differences can still be established: globulins contain glycogen and are less rich in sulfur than albumins. The presence of phosphorus noted by some authors should rather be regarded as an admixture of phosphatides adsorbed by the colloidal particles of globulins. The latter apparently are larger particles than those of albumins, possess a lower degree of dispersion and correspondingly a lower stability in solution, and precipitate more easily from it. There is an opinion that the globulins of blood represent one of the stages of degradation of the body's proteins: cellular proteins, upon disintegration, first give relatively large globulin particles, and upon further reduction of the particles through a series of continuous transitions, proteins with the properties of albumins are obtained. Being the most labile part of blood proteins, globulins apparently play an important role in immunity phenomena. During various infections, during artificial immunization, during starvation, and during processes accompanied by intensified cellular disintegration, the amount of globulins in serum can increase significantly, reaching 80% of the total amount of proteins instead of 20–25%. Immune antibodies, as well as normal hemagglutinins, precipitate together with globulins upon the latter's precipitation. The increased lability of globulins, conditioned by a change in the size of their particles, their charge, or the formation of adsorption complexes with lipoids, explains the appearance of RW, as well as a number of precipitation reactions in syphilis (see Wassermann reaction). The rate of erythrocyte precipitation has also been attempted to be linked with the relative change in the content of globulins. The methodology for quantitative determination of globulins is based on their precipitation by neutral salts, chiefly sulfates of sodium and ammonium. The precipitates obtained coagulate by boiling and, after removal of the salt, are either weighed or their nitrogen is determined by Kjeldahl. In the study of blood, the most important is not so much the determination of the absolute amount of globulins as the ratio of globulins to albumins (the so-called protein coefficient). This is achieved by parallel determination of viscosity and refractive index. With the help of special tables based on these two data, the desired ratio can be established. Another method was proposed by Rusznyak: in one strongly diluted serum sample all proteins (albumin+globulin) are precipitated by complete saturation with (NH₄)_gS0₄, and in another analogous sample only globulins are precipitated by half-saturation with the same salt. By comparing the intensity of the resulting turbidity in a nephelometer, the percentage ratio of globulins and albumins can be determined. From individual globulins one can mention: thyroglobulin, isolated from the thyroid gland, contains iodine; alpha- and beta-crystallins—globulins from the lens; serum globulin; ovoglobulin and lactoglobulin—in chicken eggs and milk; one of the proteins of muscle tissue, myosin, also belongs to globulins; to the globulin group one can also refer blood fibrinogen.