=370. Nature of Nitrogenous Bodies.=—The nitrogenous bodies, valuable
as foods, belong to the general class of proteids and albuminoids. They
are composed chiefly of carbon, hydrogen, oxygen, sulfur and nitrogen.
Some of them, as lecithin and nuclein, contain phosphorus instead of
sulfur, but these resemble the fats rather than the proteids.
Nitrogenous organic bodies of the class mentioned above are designated
by the general name proteids. The term albumin is restricted in a
physiological sense to a certain class of proteids. The term albuminoid
is often used synonymously, as above, for proteids, but, more strictly
speaking, it should be reserved for that class of bodies such as
gelatin, mucin, keratin and the like, not really proteids, but,
nevertheless, closely resembling them.[337] In chemical composition the
proteids are characterized by the relative constancy of their nitrogen
content, the mean percentage of this element being about sixteen, but
varying in some instances more than two units from that number.
=371. Classification of Proteids.=—Many classifications of the
proteids have been given based on physical, chemical and physiological
characteristics. In respect of origin, they are divided into two great
classes, _viz._, vegetable and animal. In respect of their physical and
chemical properties the following classification of the proteids may be
made.[338]
_Albumins._—These are proteids soluble in water and not precipitated
from their aqueous solutions by sodium chlorid or magnesium sulfate.
They are easily coagulated by heat and are represented by three great
classes, _viz._, egg-, serum-, and lactalbumin.
Egg albumin occurs in the white of egg; serum albumin is found in the
serum of the blood. Vegetable albumins have been prepared from wheat,
rye, potatoes, and papaws. (_Carica Papaya_). These vegetable albumins
are coagulated by heat at about 70° and are not precipitated by the
salt solutions named above, nor by acetic acid. The myrosin of mustard
seeds also resembles vegetable albumin.
_Globulins._—These bodies are insoluble in water, soluble in dilute
solutions of neutral salts, but precipitated therefrom by saturation
with sodium chlorid or magnesium sulfate. They are coagulated by heat.
Among others belonging to this group are serum globulin, fibrinogen,
myosin, crystalin, and globin.
Serum globulin is found in the serum of blood; cell globulin is found
in lymph cells; fibrinogen occurs in the blood plasma; plasmin, in
blood plasma; myosin, in dead muscles; vitellin, in the yolk of eggs;
crystalin, in the lens of the eye; haemoglobin, in the red pigment of
the blood; haemocyanin, in the blood of certain low grade animals.
Vegetable globulins are found in the cereals, leguminous plants,
papaws and other vegetables, and are divided into two groups, myosins
and paraglobulins. The vegetable myosins coagulate at from 55° to
60° and are precipitated from a saline solution by removing the salt
by dialysis. In this form, however, they lose their true nature as
globulins, becoming insoluble in weak saline solutions.
The vegetable paraglobulins are coagulated at from 70° to 75°.
Vegetable vitellin, which is not included in this classification, can
be obtained in a crystalline form and of remarkable purity.[339]
_Albuminates._—This name is given to the compounds of the proteids with
metallic oxids or bases, and also to acid and alkali albumins. They
are insoluble in water or dilute neutral salts, but easily soluble in
strong acids or alkalies. Casein is a type of this group.
Acid albumin is made from egg albumin by treatment with hydrochloric
acid; alkali albumin is formed in egg albumin by the action of a dilute
alkali; trinitroalbumin is formed from dry albumin by treatment with
nitric acid; casein or caseinogen is the chief proteid in milk.
The chief vegetable albuminates are legumin and conglutin. Legumin
is a vegetable casein and occurs chiefly in peas, beans and other
leguminous seeds. It is prepared by extracting the meal of the seeds
mentioned with dilute alkali, filtering the extract, precipitating with
acetic acid, washing the precipitate with alcohol, and drying over
sulfuric acid. Treated with sulfuric acid it yields leucin, tyrosin and
glutamic and aspartic acids. Conglutin is prepared in a similar manner
from almonds.
It is probable that these bodies do not exist as such in the fresh
seeds in question but are produced therein from the other proteids
by the alkali used in extraction. A further description of vegetable
proteids will be found in the special paragraphs devoted to the study
of these bodies in the principal cereals.
_Proteoses._—This name is applied to proteids which are not coagulated
by heat, but most of them are precipitated by saturated solutions of
neutral salts. They are also precipitated by nitric acid. They are
formed from other proteids by the action of proteolytic ferments. The
albumoses represent this group.
Protoalbumose is soluble in distilled water and weak saline solutions
and is precipitated by mercuric chlorid and copper sulfate.
Heteroalbumose is insoluble in distilled water, but soluble in weak
saline solutions, from which it separates when the salts are removed
by dialysis. Deuteroalbumose is soluble in distilled water and saline
solutions and is not precipitated on saturation with sodium chlorid. It
is thrown out by mercuric chlorid but not by copper sulfate.
Vegetable proteoses are known as phytalbumoses, two of which have been
found in the juice of the papaw mentioned above. They have also been
found in cereals.
_Peptones._—These bodies are very soluble in water but are not thrown
out by heat, by saturation with neutral salts, nor by nitric acid. They
are completely precipitated by tannin and by strong alcohol.
The peptones are the only soluble proteids which are not precipitated
by saturation with ammonium sulfate. The principal animal varieties are
hemi- and anti-peptones. These forms of proteids do not appear to exist
as such in vegetable products but are produced in large quantities by
treating other proteids with pepsin or pancreatin. In sprouting plants,
there appears to be a widely diffused ferment capable of converting
the proteids of the cotyledons into peptonoid bodies and thus fitting
them for entering the tissues of the new plant.
_Insoluble Proteids._—This class includes a miscellaneous collection
of nitrogenous bodies not belonging to any of the definite groups
already mentioned. Fibrin and gluten are types of these insoluble
bodies. Fibrin is formed from the fibrinogen of fresh blood and causes
coagulation. When washed free of red blood corpuscles it is a white
elastic solid. It is insoluble in water and is converted into albumoses
and peptones by trypsin and pepsin. It swells up when treated with
a very weak one-tenth per cent solution of hydrochloric acid and
dissolves to acid albumin when heated therewith.
Gluten is the most important of the insoluble vegetable proteids and
forms the chief part of the nitrogenous constituents of wheat. It is
readily prepared by washing wheat flour in cold water, as will be
described further on. It is probably a composite body formed by the
process of extraction from at least two proteid bodies existing in
wheat. When dried it forms a horny elastic mass of a yellow-gray color.
Gluten is composed of two bodies, one soluble the other insoluble
in alcohol. The part insoluble in alcohol has been called vegetable
fibrin, and the soluble part is subdivided into two portions, one
unicedin or vegetable unicin, and the other glutin (gliadin) or
vegetable gelatin. Gluten, according to some authorities, does not
properly exist in wheat flour, but is formed therein by the action
of water and certain ferments from free existing proteids. A better
explanation of the composition of gluten is that of Osborne, which will
be given further on.
=372. Albuminoids.=—In this paragraph the term albuminoids is not
employed as synonymous with proteids but as characteristic of a class
of bodies nearly resembling them, but, nevertheless, differing from
them in many important particulars. Following is an abstract of their
classification as given in Watt’s dictionary.[340]
_Collagen._—The nitrogenous portions of connective tissues are largely
composed of collagen. By boiling water it is converted into gelatin.
It may be prepared from tendons as follows: The tendinous tissues are
shredded as finely as possible and extracted with cold water to remove
the soluble proteids. Thereafter they are subjected for several days to
the action of lime water, which dissolves the cement holding the fibers
together. The residual insoluble matter is washed with water, weak
acetic acid, and again with water. The residue is chiefly collagen,
mixed, however, with some elastin and nuclein. With dilute acids and
alkalies collagen swells up after the manner of fibrin. The organic
nitrogenous matter of bone consists largely of collagen, which is
sometimes called ossein.
_Gelatin._—When the white fibers of collagen, obtained as above, are
subjected to the action of boiling water or of steam under pressure
they dissolve and form gelatin. Isinglass is a gelatin made from the
swimming bladder of the sturgeon or other fish. Glue is an impure
gelatin obtained from hides and bones. Pure gelatin may be prepared
from the commercial article by removing all soluble salts therefrom by
treatment with cold water, dissolving in hot water and filtering into
ninety per cent alcohol. The gelatin separates in the form of white
filaments and these are removed and dried. Gelatin is insoluble in
cold but soluble in hot water. It is insoluble in alcohol, ether and
chloroform. Its hot aqueous solutions deflect the plane of polarized
light to the left. Its gyrodynat varies with temperature and degree of
dilution and is also influenced by acids and alkalies. At 30° it is
[α]_{D}³⁰° = -130.
Gelatin is not precipitated by acetic acid nor lead acetate solution,
in which respect it differs from chondrin.
If boiled for a day, or in a short time if heated to 140° in a sealed
tube, gelatin loses its power of setting and is split up into two
peptonoid bodies, semi-glutin and hemi-collin. Gelatin is easily
digested but cannot take the place of other proteids in nutrition.
_Mucin._—This albuminoid, together with globulin, forms the principal
part of connective tissue. It is also present in large quantities in
mucus and is the chief lubricant of mucous membranes. It is extremely
difficult to prepare mucin in a state of purity, and it is not certain
that it has ever been accomplished. It is precipitated but not rendered
subsequently insoluble by sodium chlorid, magnesium sulfate and
alcohol. When boiled with sulfuric acid it yields leucin and tyrosin
and, with caustic soda, pyrocatechin.
_Met- and Paralbumin._—Metalbumin is a form of mucin and differs from
paralbumin by giving no precipitate when boiled. Both bodies yield
reducing sugars when boiled with dilute sulfuric acid.
_Nuclein._—The nitrogenous matters which form the nuclei of the
ultimate cells are called nuclein. Nuclein resembles mucin in many
physical properties but contains phosphorus. It is also, like mucin,
resistant to pepsin digestion. The nuclein of eggs and milk probably
contains iron. Nuclein is found also in cells of vegetable origin and
in yeast and mildew.
_Nucleoproteids._—These are bodies which yield both nuclein and albumin
when boiled with water or treated with dilute acids or alkalies. Many
nucleoproteids have the physical properties of mucus and the sliminess
of the bile and of the synovial liquid is due to them. They are the
chief nitrogenous constituent of all protoplasm.
_Chondrin._—Chondrin is obtained from cartilage by boiling with water.
The solutions of chondrin set on cooling in the manner of gelatin. They
are precipitated by the same reagents used for throwing out gelatin and
mucin. Chondrin is also levorotatory. By some authorities chondrin is
regarded as a mixture of gelatin and mucin.
_Elastin._—The elastic fibers of connective tissue are composed of
this material. It can be prepared from the neck muscles by boiling
with ether and alcohol to remove fats and then for a day and a half
with water to extract the collagens. The residue is boiled with strong
acetic acid and thereafter with strong soda until the fibers begin to
smell. It is then treated with weak acetic acid and for a day with
dilute hydrochloric acid. The acid is removed by washing with water
and the residue is elastin. There is no solvent which acts on elastin
without decomposing it. It is digested by both pepsin and trypsin with
the formation of peptones.
_Keratin._—This nitrogenous substance is found chiefly in hairs,
nails, and horns. It is essentially an alteration proteid product due
to peripheral exposure. It is prepared by digesting the fine ground
material successively with ether, alcohol, water and dilute acids. The
residue is keratin. An imperfect aqueous solution may be secured by
heating for a long time under pressure to 200°. It is also dissolved
by boiling the materials mentioned above with alkalies, and when the
solution thus obtained is treated with water, hydrogen sulfid is
evolved, showing that the sulfur of the molecule is loosely combined.
Horn swells up when treated with dilute acetic acid and dissolves in
the boiling glacial acid. When treated with hot dilute sulfuric acid it
yields aspartic and volatile fat acids, leucin and tyrosin. Keratin,
when burning, gives off a characteristic odor as is perceived in
burning hair.
_Other Albuminoids._—Among the albuminoids of less importance may be
mentioned neurokeratin found in the medullary sheath of nerve fibers;
chitin occurring in the tissues of certain invertebrates; conchiolin,
found in the shells of mussels and snails; spongin, occurring in
sponges; fibroin forming silk and spiders webs; and hyalin or hyalogen
found in edible birds’ nests.
The nitrogenous bases in flesh which are soluble in cold water,
_viz._, kreatin, kreatinin, carnin, sarkin and xanthin are not classed
among the albuminoid bodies, since they have a much higher percentage
of nitrogen than is found in true proteid bodies, and are further
differentiated from them by the absence of sulfur.
=373. Other Forms of Nitrogen.=—In addition to the proteids and
albuminoids mentioned above, agricultural products may contain nitrogen
in the form of ammonia, amid nitrogen and nitric acid. The quantities
of nitrogen thus combined are not large but often of sufficient
magnitude to demand special study. In general, these bodies belong to
transition products, representing stages in the transfer of nitrogen
from the simple to complex forms of combination, or the reverse.
For instance, the nitrogen which finally appears in the proteids
of a plant has entered its organism chiefly as nitric acid, and
the nitric acid which is found in a vegetable product is therefore
a representative of the quantity of unabsorbed nitrogen present in
the tissues at the moment when the vital activity of the plant is
arrested. In some instances, it is found that the absorption of
nitrates by vegetable tissues takes place in far larger quantities than
is necessary for their nutrition, and in these cases the excess of
nitrates accumulates, sometimes to a remarkable extent. In a case cited
in the reports of the Kansas Agricultural Experiment Station, where
Indian corn was grown on ground which had been used for a hog pen, the
quantity of potassium nitrate found in the dried stalks was somewhat
remarkable. When one of the stalks was cut in two and tapped lightly
upon a table, crystals of potassium nitrate were easily obtained in the
form of fine powder. On splitting the cornstalk the crystals in the
pith could be seen without the aid of a microscope. On igniting a piece
of the dried stalk it burned rapidly with deflagration. The percentage
of potassium nitrate in the dried material was 18.8. Cattle eating this
fodder were poisoned.[341]
In preserved meat products large quantities of oxidized nitrogen are
often found, and these come from the use of potassium nitrate as a
preserving and coloring agent. Ammonia is rarely found in vegetable
tissues in greater quantities than mere traces, but may often exist in
weighable amounts in animal products.
Amid nitrogen is found rather constantly associated with proteid
matters in vegetable products. Asparagin and glutamin are instances
of amid bodies of frequent occurrence. Betain and cholin are found in
cottonseed.
The occurrence of nitrogen, in the form of alkaloids, is of interest
to agricultural chemists in this country, chiefly from its presence
as nicotin in tobacco and from a toxicological point of view, but
in other localities the production of alkaloids, as for instance in
opium, tea and coffee, is a staple agricultural industry. The methods
of separating and determining these forms of nitrogen will be given
further on. This description can evidently not include an extended
compilation of the methods of separating and determining alkaloidal
bodies, with the exception of those with which the agricultural
analyst will be called upon frequently to deal, _viz._, nicotin and
caffein and nitrogenous bases such as betain and cholin.