THE RUMFORD KITCHEN LEAFLETS. 
No. 12. 
SOME POINTS IN THE CHEMISTRY OF PROTEIDS 
Written for the Rumford Kitchen by John J. Abel. 
The proteids and substances so closely allied as to be classed 
with them are, next to water, the chief constituents by weight of 
the human body. Roughly speaking, the human body is sixty- 
five per cent, water, fifteen per cent, proteids and proteid-like 
bodies, while the remaining twenty per cent, is made up mainly 
of fat, a few per cent, of mineral constituents, and smaller quan- 
tities of other substances, such as the carbohydrates and extrac- 
tives. When separated by chemical means from the salts, fats, 
and other substances with which they are closely bound up in 
living tissue, the proteids are seen to be non-crystalline in form, 
and easily affected by heat and chemical agents. In the body it- 
self they must be thought of as existing either in a kind of solu- 
tion in its fluids, as for example in blood, or in a more viscous 
condition in its tissues of firmer structure, but as soon as life is 
extinct many of them take on the more solid or coagulated form. 
Many varieties of proteids are to be found in the various tissues. 
Thus, from the muscles may be isolated myosin and muscle-albu- 
men, from the blood, serum-albumen, fibrinogen, paraglobulin, and 
haemoglobin ; from the brain, neurokeratin and nuclein ; from con- 
nective tissue and bone, collagen or gelatine-giving material; 
Copyright, 18U3, uy Ellen II. Richards. from the network of glandular structures, elastine and reticulin; 
from the hair and skin, keratin; and from the milk, casein. 
These proteids and proteid-like bodies are all ultimately derived 
from the proteids built up by plants out of very simple inorganic 
constituents. The animal organism has the power of splitting up 
proteids that have been consumed as food, and of modifying them 
in many ways, so that varieties very different from those originally 
digested are deposited in the tissues; but it can not, like the plant, 
build up proteid anew out of, we might almost say, the chemical 
elements themselves. The animal body, then, takes its proteid 
either in the form of plant proteid or plant proteid previously 
transformed into animal proteid. Plant and animal proteids, 
though closely related, are by no means identical. 
Only a few chemical elements enter into the composition of 
proteids. All contain carbon, hydrogen, oxygen, and nitrogen, 
and most of them also sulphur; several contain phosphorus in 
addition, and a few, iron or copper. Their general percentage 
composition varies within the following limits: 
Carbon ...... 50.0 — 55.0 % 
Hydrogen G.8 — 7.3 % 
Nitrogen 15.4 — 18.2 % 
Sulphur 0.4 — 5.0 % 
(Phosphorus 0.42 — 0.85 °/L) 
Oxygen 20,8 — 24.1 % 
With the help of such analytical data, chemists are wont to 
establish by calculation what is known as the lowest empirical 
molecular formula of a substance, and by taking into account 
certain physical and chemical data arrived at in other ways, this 
empirical formula is changed into what is called a rational for- 
mula, one representing the molecular magnitude and the molecular 
structure of the substance analyzed. For proteids, however, not 
even a definite empirical formula has been arrived at in spite of 
the innumerable ultimate analyses that have been made of various 
proteids, and the chief reasons for this are the difficulty of de- termining the chemical individuality of a given proteid or of its 
compounds, and the uncertainties introduced by minute analytical 
errors in the case of substances presumably containing so large a 
number of atoms in their molecule. An older formula, based on 
the analysis of purified white of egg that had been treated with 
strong caustic soda, and representing the lowest empirical for- 
mula, is C72 H112 N18 S O22. Other analyses of so-called crystalline 
proteids, or more properly speaking, of loose compounds of 
proteids and mineral constituents, have led to formula? represent- 
ing very high molecular weights. Here, too, such difficulties 
present themselves that we must confess that we have no notion 
whatever of the true molecular weight of the proteids, and it goes 
without saying that we are equally in the dark as to their molec- 
ular constitution. So many chemists of repute, however, are 
working in this field that we may hope to see in the near future 
light thrown on this important question of biology. 
In the absence of chemical data of a higher order, it has become 
necessary in the interests of medicine, biology, and other sciences, 
to classify these bodies in accordance with their behavior toward 
heat, acids, alkalis, metallic compounds, distilled water, dilute 
saline solutions, etc. Thus, serum-albumen, one of the chief 
proteids of the blood and egg-albumen are classed together, for 
they are both soluble in distilled water and in dilute solutions of 
common salt, and both coagulate on being heated to from 50°— 
80° C. A closely allied class to which casein belongs are insolu- 
ble in distilled water and in weak solutions of common salt, but 
soluble in weak alkaline solutions and not coagulable when their 
pure alkaline solutions are heated. Two other classes constitut- 
ing the albumoses and peptones result from the action of the 
digestive juices on the proteids of our food. These are in general 
extremely soluble in water and behave very differently toward 
precipitating reagents from those already named. 
By the application of such chemical tests the proteids proper are 
classified into albumins, globulins, nucleo-albumins, albumin-, 
ates, fibrins, albumoses, and peptones. In addition to these there are other constituents of the body that we have referred to as 
proteid-like substances, which, while differing among themselves 
more decidedly than the groups that we have just named, yet 
resemble the proteids so much that they must be brought into 
connection with them. Some of these allied proteids contain a 
non-proteid residue like sugar in close combination with them. 
These are known as mucin, hannoglobin, keratin, elastin, reticu- 
lin, collagen, gelatin, etc., and still other representatives are 
found widely distributed in the animal world. 
Not only are the proteids that have been absorbed from our 
food variously modified within the body and built up into its struc- 
ture as has already been intimated, but they are also constantly 
undergoing destruction as part of the material that furnishes the 
various forms of energy required. The final or refuse products 
of this destructive process are water, carbonic acid, and certain 
nitrogenous substances, chief among which is one called urea. 
This substance is excreted by the kidneys, and in the case of 
adults about an ounce is eliminated every twenty-four hours. But 
a large number of other nitrogenous substances, all derived from 
proteids, and all playing a more or less important role in the up- 
ward or downward chemical changes to which proteids are sub- 
jected, are also met with in the tissues and fluids of the body. 
Some of these other nitrogenous substances are uric acid, xanthin, 
sarkin, guanin, adenin, leucin, tyrosin, kreatin, and taurin. Many 
of these are, furthermore, of interest since we take them in con- 
siderable quantities every day with our meats, soups, etc., as 
natural physiological stimulants. Meat extracts, such for in- 
stance as Liebig’s extract, afford a most convenient source for 
their preparation and study, notably for kreatin and sarkin. The 
active principles of tea, coffee, and cocoa are chemically inti- 
mately related to those nitrogenous “ extractives” of meat to 
which the name of the xanthin group has been given. 
The chemical steps involved in the formation of these various 
substances within the body tissues are still largely a matter of 
mystery. In order to elucidate the manner in which proteids may 
4 break down or may be reconverted in the body, chemists have 
treated them with strong acids, alkalis, and other chemical agents 
under various conditions such as high temperatures and high 
pressure. The' artificial decomposition of proteids in the lab- 
oratory has yielded us many of the intermediate products referred 
to, and, taken into account along with the results of physiologi- 
cal experimentation, has furnished us with at least a rough 
sketch of the chemical history of proteids from the moment of 
their entrance into the alimentary canal to the appearance of their 
fragments lodged in the tissues or circulating in the fluids of the 
body. To attempt this sketch here would however take us be- 
yond the scope of this leaflet.