LESSON III

THE ALBUMINATES AND ALBUMINOIDS

1. Action of Acids and Alkalis on Albumin.-Take three test-tubes and label them A, B, and C.

In each place an equal amount of diluted egg-white, similar to that used in the last lesson.

To A add a few drops of 0.1-per-cent. solution of caustic potash.

To B add the same amount of 0.1-per-cent. solution of caustic potash.

To C add a rather larger amount of 0.1-per-cent. sulphuric acid.

Put all three into the warm bath1 at about the temperature of the body (36-40° C.)

After five minutes remove test-tube A, and boil. The proteid is no longer coagulated by heat, having been converted into alkali-albumin. After

FIG. 8.-Simple warm bath, as described in footnote.

cooling, colour with litmus solution and neutralise with 0.1-per-cent. acid. At the neutral point a precipitate is formed which is soluble in excess of either acid or alkali.

Next remove B. This also now contains alkali-albumin. Add to it a few

1 A convenient form of warm bath suitable for class purposes may be made by placing an ordinary tin pot half full of water over a bent piece of iron which acts as a warm stage as in the figure. The stage is kept warm by a small gas flame. Such a warm bath may be placed between every two or three students.

drops of sodium phosphate, colour with litmus, and neutralise as before. Note that the alkali-albumin now requires more acid for its precipitation than in A, the acid which is first added converting the sodium phosphate into acid sodium phosphate.

Now remove C from the bath. Boil it. Again there is no coagulation, the proteid having been converted into acid-albumin, or syntonin. After cooling, colour with litmus and neutralise with 0.1-per-cent. alkali. At the neutral point a precipitate is formed soluble in excess of acid or alkali. (Acid-albumin is formed more slowly than alkali-albumin, so it is best to leave this experiment to the last.)

2. Take some gelatin and dissolve it in hot water. On cooling, the solution sets into a jelly (gelatinisation).

3. Add a few drops of acetic acid to some saliva. A stringy precipitate of mucin is formed.

4. A tendon has been soaked for a few days in lime water. The fibres are not dissolved, but they are loosened from one another owing to the solution of the interstitial or ground substance by the lime water. Take some of the lime-water extract and add acetic acid. A precipitate of mucin is obtained. The fibres themselves consist of collagen, which yields gelatin on boiling.

ALBUMINATES

Albuminates are compounds of proteid with mineral substances. Thus, if a solution of copper sulphate is added to a solution of albumin, a precipitate of copper albuminate is obtained. Similarly, by the addition of other salts of the heavy metals, other metallic albuminates are obtainable.

The albuminates which are obtained by the action of dilute acids and alkalis on either albumins or globulins are, however, of greater physiological interest, and it is to these we shall chiefly confine our attention. The general properties of the acid-albumin, or syntonin, and the alkali-albumin which are thereby respectively formed will be gathered from the practical exercise which stands at the head of this lesson. They are insoluble in pure water, but are soluble in either acid or alkali, and are precipitated by neutralisation unless disturbing influences like the presence of sodium phosphate are present. It may also be added that, like globulins, they are precipitated by saturation with such neutral salts as sodium chloride and magnesium sulphate. They are not coagulated by heat.

A variety of alkali albumin (probably a compound containing a large quantity of alkali) may be formed by adding strong potash to undiluted white of egg. The resulting jelly is called Lieberkühn's jelly.

Caseinogen, the chief proteid in milk, was at one time regarded as being a native alkali-albumin. It certainly is precipitated by acid, but we shall find that there are several reasons why it is no longer considered an alkali-albumin.

ALBUMINOIDS

The albuminoids are a group of substances which, though similar to the proteids in many particulars, differ from them in certain other points. The principal members of the group are the following:

1. Collagen, the substance of which the white fibres of connective tissue are composed. Some observers regard it as the anhydride of gelatin. 2. Ossein. This is the same substance, derived from bone.

3. Gelatin. This substance is produced by boiling collagen with water. It possesses the peculiar property of setting into a jelly when a solution made with hot water cools. It gives most of the proteid colour tests. Many observers state, however, that it contains no sulphur. On digestion it is like proteid converted into peptone-like substances, and is readily absorbed. Though it will replace in diet a certain quantity of proteid, acting as what is called a 'proteidsparing' food, it cannot altogether take the place of proteid as a food. Animals fed on gelatin instead of proteid waste rapidly. Chondrin, the very similar substance obtained from hyaline cartilage, appears to be a mixture of gelatin with mucinoid materials.

4. Mucin. This is a widely-distributed substance, occurring in epithelial cells, or shed out by them (mucus, mucous glands, goblet cells); and in connective tissue where it forms the chief constituent of the ground substance or intercellular material.

The mucin obtained from different sources varies in composition and reactions. There are probably several mucins they all agree in the following points

n

FIG. 9.- Diagram of a cell: p, protoplasm composed of spongioplasm and hyaloplasm; n, nucleus with intranuclear network of chromatin or nuclein; and n', nucleolus. (Schäfer.)

(a) Physical character. Viscid and

tenacious.

(b) Precipitability from solutions by acetic acid: they all dissolve in dilute alkalis like lime water.

(c) They are all compounds of a proteid with a carbohydrate called animal gum, which by treatment with dilute

mineral acid can be hydrated into a reducing but non-fermentable sugar.

5. Elastin. This is the substance of which the yellow or elastic fibres of connective tissue are composed. It is a very insoluble material. The sarcolemma of muscular fibres and certain basement membranes are very similar.

6. Nuclein, the chief constituent of cell-nuclei. A similar substance

is also found in milk and yolk of egg.

Its physical characters are

something like mucin, but it differs chemically in containing a high percentage of phosphorus. Nuclein appears to be identical with the chromatin of histologists (see fig. 9).

7. Nucleo-albumin.-Compounds of proteids with nuclein. They are found in the protoplasm of cells. Caseinogen of milk and vitellin of egg-yolk are similar substances. In physical characters they often closely simulate mucin; in fact, the nucleo-albumin of bile is still frequently called mucin. They may be distinguished from mucin by the fact that they do not yield a reducing sugar on treatment with mineral acid; they yield on gastric digestion not only peptone but also an insoluble residue of nuclein which is soluble in alkalis, precipitable by acetic acid from such a solution, and contains a high percentage of phosphorus.

Some of the nucleo-albumins also contain iron, and it is probable that the normal supply of iron to the body is contained in the nucleoalbumins, or hæmatogens (Bunge), of plant and animal cells.

8. Keratin, or horny material, is the substance found in the surface layers of the epidermis, in hairs, nails, hoofs, and horns. It is very insoluble, and chiefly differs from proteids in its high percentage of sulphur. A similar substance, called neurokeratin, is found in neuroglia and nerve fibres. In this connection it is interesting to note that the epidermis and the nervous system are both formed from the same layer of the embryo-the epiblast.

LESSON IV

MILK, FLOUR, BREAD

A. Milk.-1. Examine a drop of milk with the microscope.

2. Note the specific gravity of fresh milk with the lactometer, and observe that the specific gravity is increased by the removal of the lightest constituent-the cream.

3. The reaction of fresh milk is usually neutral or slightly alkaline.

4. Warm some milk in a test-tube to the temperature of the body, and add a few drops of rennet. After standing, a curd is formed from the conversion of caseinogen, the chief proteid in milk, into casein. The casein entangles the fat globules; the liquid residue is termed whey. No curd forms if the rennet solution is previously boiled. Heat kills ferments.

5. To another portion of warm milk diluted with water add a few drops of strong acetic acid. A lumpy precipitate of caseinogen entangling the fat is formed.

6. Filter off the curd, and in the whey test for lactose or milk sugar by Trommer's test (see Lesson I.); for lactalbumin by boiling, or by Millon's reagent (see Lesson II.); and for earthy (that is, calcium and magnesium) phosphates by ammonia, which precipitates these phosphates.

7. Fat (butter) may be extracted from the curd by shaking it with ether; on evaporation of the ethereal extract the fat is left behind, forming a greasy stain on paper.

8. Caseinogen, like globulins, is precipitated by saturating milk with sodium chloride or magnesium sulphate, but differs from the globulins in not being coagulated by heat. The precipitate produced by saturation with salt floats to the surface with the entangled fat, and the clear salted whey is seen below after an hour or two.

B. Flour.-Mix some wheat flour with a little water into a stiff dough. Wrap this up in a piece of muslin and knead under a tap or in a capsule of water. The starch grains come through the holes in the muslin (identify by iodine test), and an elastic sticky mass remains behind. This is a proteid called gluten (identify by xanthoproteic or Millon's reaction).

C. Bread contains the same constituents as flour except that some of the starch has been converted into dextrin and dextrose during baking (most flours, however, contain a small quantity of sugar). Extract bread with cold water, and test the extract for dextrin (iodine test) and for dextrose (Trommer's test). If hot water is used starch also passes into solution.

THE PRINCIPAL FOOD-STUFFS

We can now proceed to apply the knowledge we have obtained of the proteids, carbohydrates, and fats to the investigation of some important foods. We do not actually use as food the various organic