63. Nucleoproteins

These are complex, phosphorus-bearing proteins that sustain an important nutritive function. They are regarded as a combination of nudein with an albumin, the nucleins being compounds of nucleic acid and albumin, and nucleic acid yielding on cleavage phosphoric acid, certain nitrogenous bases known as purins, and in all cases a carbohydrate.

The nucleoproteins are associated with the nuclei of the cells that make up both plant and animal tissues, and consequently are found in the flesh of animals that is used for food. They are relatively abundant in glandular tissues such as the spleen, pancreas, thymus gland, and liver. The spermatozoa masses of fishes are especially rich in these bodies. Because certain bases known as purins which arise from the cleavage of nucleoproteins are regarded as the progenitors of uric acid, persons with uric acid tendency are advised to avoid eating certain animal foods such as beef and liver, or any others known to contain these compounds. Experiments show that the feeding of certain tissues rich in nucleoproteins increases the output of uric acid, while adding to the diet a large amount of purin-free proteins such as albumin does not have this effect.

64. Glycoproteins (Glucoalbumins)

These are bodies that upon cleavage are decomposed into a protein and a carbohydrate. The best-known glycoproteins are the mucins that are secreted, for instance, by the mucous membranes of the air passages and of the alimentary canal and by certain glands such as the salivary. Certain of these compounds contain phosphorus, and others do not.

55. Phosphoproteins (Nucleoalbumins)

Like the nucleoproteins, these compounds contain phosphorus, but on cleavage do not yield the purin bases that under certain conditions are to be avoided. The best-known phosphoprotein is the casein of milk, a compound exceedingly important in human nutrition, especially with the young.

This compound is a secretion of the mammary gland of many species of animals, and doubtless originates in the contents of the gland cells. As will be seen later, the casein from different species of mammals differs somewhat in chemical and physical properties. Casein is insoluble in water, but exists in milk in suspension. It is not coagu- lated by heat, but curdles when a weak acid is added to milk, as, for instance, vinegar. The same result is produced by a generous quantity of common salt. When milk is ingested into the human stomach, the casein coagulates (the milk curdles) through the action of a ferment in the gastric juice (see p. 90), and this coagulation is unlike with milk from different species. The action of this ferment on casein is utilized in cheese making in the development of a curd which, with its inclosed fat, is separated from the whey and pressed into compact masses and later allowed to undergo certain changes due to other ferments.

Other phosphoproteins exist, one being the vitellin in the yolk of eggs, which, as prepared, contains lecithin. (See p. 82.)

56. Haemoglobins

Blood contains a peculiar compound known as hamoghbin. When decomposed, it separates into a protein, globin, and a coloring matter (pigment), which, when charged with oxygen, is called hoematin. This haemoglobin in the blood of mammals contains, besides carbon, nitrogen, oxygen, and hydrogen, sulfur and iron. The latter varies in per cent from.34 to.48, and sustains an essential relation to the functions of the blood. The blood pigment has the property of taking up and releasing oxygen with great readiness, carry-ing its load of oxygen out of the lungs, giving it up to oxidation processes in various parts of the body, and bringing to the lungs in its place the resulting carbon dioxid to be discharged into the air. The blood changes color with the acquisition and loss of the oxygen.

57. Lecithoproteins

From the yolk of eggs, the mucous membranes, and the kidneys, and doubtless from other sources, are obtained a conjugated protein contain-. ing lecithin. The constitution and special function of this body are not well understood.