46. The Globulins

It is fully recognized that when plant and animal tissues are treated with water, but a small part of the proteins dissolve. If, however, we add to the water a mineral salt, especially common salt (sodium chloride), sufficient to secure a 10 per cent solution, an additional and considerable amount of protein may be extracted. These compounds are called globulins, and differ from the albumins in being insoluble in pure water and in a saturated solution of certain mineral salts, such as sodium chloride. The globulins form an important part of the protein content of plants and of animal tissues, both in quantity and in having a maximum nutritive usefulness. 47. Plant globulins. - In plants these proteins seem to be especially abundant and widespread. Our most recent and most reliable knowledge of plant proteins comes from investigations conducted in the laboratory of the Connecticut Agricultural Experiment Station, chiefly by Osborne. In these researches the seeds of many species of agricultural plants were studied, all of which were found to contain globulins. In some the proteins consisted largely of these compounds. The percentage content in certain seeds was determined approximately:

Table XIV. Globulins In Certain Seeds

Kidney bean .... 20.0

Cottonseed meal . . . 15.8

Peas....... 10.0

Lupin....... 26.2

Per Cent Per Cent

Wheat ... 0.6

Lentil . . . 13.0

Horse bean . . 17.0 Maize ... 0.4

Soy bean . Chiefly globulin

The seeds of the legumes, as a rule, contain the largest proportion of these proteins, the cereal grains having only a very small part of their protein in this form.

From present knowledge, many seeds appear to have characteristic globulins which differ among themselves in their chemical properties. These have been given names derived from the general names of the species in which they are found. Thus we have amandin in almonds, avenalin in oats, corylin in walnuts, excelsin from the Brazil-nut, phaseolin in several species of beans, glycin in the soy bean, maysin in maize, vicilin in horse beans, lentils, and peas, vignin in the cow-pea, and tuberin in the potato. One globulin called edestin appears to be quite generally distributed in the seeds of agricultural plants, having been found in a larger number than any other protein yet discovered, including all the cereals? castor bean, cottonseed, flaxseed, hemp, squash, and sunflower, though it is not abundant in any of these.

48. Animal Globulins

The animal globulins of which we have definite knowledge are those that exist in the muscle and in the blood. The names which some of them bear are myosin, fibrinogen, and paraglobulin. If finely divided, well-washed muscle (lean meat) is treated with a 10 per cent salt solution, first by rubbing it in a mortar with fine salt, and then adding enough water to secure the proper strength of solution, a globulin is dissolved to which the name myosin has been given. The view has been generally accepted that this compound does not exist as such in living muscle, but forms there by coagulation upon the death of the animal. This change has been looked upon as similar to the coagulation of blood through the formation of fibrin, and is regarded as the explanation of the stiffening of dead muscles (rigor mortis). The theory has been broached that a "mother" substance exists in the living muscle, from which myosin is formed in much the same way as fibrin is developed in clotting blood from a preexisting body, but no single view as to exactly what occurs is fully accepted. There is, nevertheless, a general agreement that rigor mortis is due to a clotting of the muscle, accompanied by marked chemical transformations, one final product being myosin. The theory is advanced that ferments are present in the muscle, to the influence of which these changes are due, and without which they do not occur.

Another prominent and remarkable globulin is the fibrinogen found in the blood. It is common knowledge that when blood is drawn from the veins and cools, it clots, - a phenomenon which is nothing more than the formation of strings of fibrin. Fibrin as such is not found in living blood, but is one of the products into which fibrinogen splits when exposed blood cools, probably because of the influence of a ferment. Stranger than all is the fact that so long as the blood is retained in the arteries and veins, even if the animal dies and grows cold, this clotting does not appear.

Serum-globulin is a collective name for several globulins, which exist in blood serum and in the other fluids of the animal body, such as lymph and its allies, including those exudations which pertain to diseased conditions, especially dropsical.

One more protein has been generally classified as a globulin, although differing in some respects from the other members of this class, and more recently is classed as a phosphoprotein. Reference is made to vitellin, which is the principal protein in the yolk of eggs. It is there intimately mixed with certain peculiar phosphorized bodies, which we shall notice later.