It is not difficult to account for the formation of such com- pounds from carbohydrate materials and ammonia. Peculiar to all is the group CH(NH 2 ).CO2H. The presence of this group is almost certainly a clue to the process by which they are produced, viz. by the action of ammonia on a keto-carboxy-acid. Much has been said above of pyruvic acid: alanine is doubtless formed from this acid by its combination with ammonia and subsequent reduction of the hydroxy-amino acid:
CH 3 CO.CO 2 H+NH 3 = CH 3 .C(OH)(NH 2 ).CO 2 H CH 3 .C(OH) (NH 2 ).CO 2 H +2H = CH 3 .CH (NH 2 ).CO 2 H
In the human circulation amino-acids are converted into oxy- acids which serve as fuels by the reverse process. As the amino-acids are optically active substances, like the glucosides, the reduction process must be directed in some way: they belong to one series corresponding to that of the natural sugars. The name amino-acid, usually given to the protein hydroclasts, is not applicable to the compounds as they exist apart, although they function as such. The names in the above list, which are those usually given, indicate that most of the compounds are basic rather than acid. As a matter of fact, owing to internal neutralization,
.CH
NH 2 COOH
.CH
NH 3 CO.O
they are all but neutral substances, yet they can act either as acid or base, according to circumstances.
Although obtainable from animal as well as vegetable pro- teins, all the compounds in the list given, with the exception of the first, are the products of plant activity alone. The office of the animal is to take to pieces the complex structures which are eaten as vegetable food; then, having conveyed them in the blood stream to various parts of the body, to reconstruct them in ap- propriate special ways. In some cases the units are built up around a phosphoric acid nucleus, particularly in cell nuclei, in brain matter and bone marrow. A number of compounds be- sides those in the list, even sugars, especially galactose and ribose, are met with proteins.
In the carbohydrates the linkage is etheric two carbon atoms are joined through the agency of an oxygen atom. In the pro- teins two carbon atoms are linked together through the agency of a nitrogen atom. An additional peculiarity is the presence of a succession of NH.CH.CO groups, each forming as it were a short link in a chain, each link carrying a side-group which may vary greatly in character and dimensions attached to the CH member. A large number of " polypeptides " have been built up in the laboratory, from amino-acids, on such a plan as the following: NH 2 .CH 2 .CO. ! NH.CH.CO ! NH.CH.CO I NH.CH.CO.OH
I I I
CH, C 3 H 7 C
Even octadecapeptides have been prepared, indeed there seems to be no theoretical limit to the number of " links " that may be included in the chain. In this field however, as in that of the carbohydrates, there is reason to believe that nature has been sparing in her selection and choice of patterns. At present, not the least clue to the patterns laid down has been obtained: at most the order followed in some of the smaller fragments ob- tained from proteins by hydrolysis has been ascertained.
The uniformity that exists could not be reached the possible permutations and combinations are so infinitely numerous if selective and directive influences were not at work, of the order of those referred to in discussing the carbohydrates. It would be easy to prepare many unit materials similar to those in use in plants and animals, but there can be no doubt they would not be assimilated as foods. If not poisonous they would either be seized on by glucose molecules and quickly emptied into the urine or got rid of as such; or they would be just thrown into the circulation and burnt up in its fires.
The great advance of modern times, since it became possible to analyse the proteins, is the recognition of the prime fact that a varied and well proportioned diet is essential, if all the structural elements required in growth are to be at disposal. Latterly the even more important discovery has been made that fresh and un- cooked foods contain minute proportions of mysterious materials
Ox Muscle
Protein.
Casein.
Lact- albumin.
Gelatin.
Wheat Gliadin.
Wheat Glutenin.
Maize Zein.
Maize Glutenin.
Edestin.
Sturin.
Glycine
2-1
o
o
19-3
o
0-9
o
0-3
3-8
Alanine
3-7
i-5
2-5
3-o
2-0
4-7
9-8
3-6
Valine
0-8
7-2
0-9
3'4
O-2
1-9
+
Leucine
11-7
9.4
19-4
6-8
6-6
6-0
19-6
6-2
20-9
Phenylalanine
3'2
3-2
2-4
I-O
2-4
2-0
6-6
3'i
Tryosine .
2-2
4-5
0-9
o
1-2
4-3
3-6
3'8
2-1
Serine
o-5
0-4
O-2
0-7
I'D
o-3
Cystine
(?)
o-5
O-O2
o-3
Proline
5-8
6-7
4-0
10-4
13-2
4-2
9-0
5'9
4-1
Hydroxyproline
o-3
6-4
2-O
Aspartic acid
4-5
1-4
I-O
1-2
0-6
0-9
i-7
0-7
4-5
Glutamic acid
15-5
15-6
IO-I
1-8
437
23-4
26-2
12-7
18-7
Tryptophanc Argmine .
+ 7-5
1-5 3-8
3-2
9-3
I-O
3'2
+
47
o 1-6
+ 7-1
+ 14-4
58-2
Lysine
7-6
6-0
9-2
5-o
O-2
1-9
o
3-o
1-7
I2-O
Histidine .
1-8
2-5
2-1
0-4
0-6
[
0-8
3-o
2-4
[2-9
Ammonia .
l-l
1-6
i-3
0-4
5-2
4-0
3-6
2-1
Total
67-5
66-5
57-o
65-4
83-0
59-72
85-4
45-7
81-9
8 3 -I
The analysis of the proteins is a difficult operation and the re- sults are usually but approximations. In the accompanying table, the results of such an analysis are set out. Those quoted are sufficient to show how complex is their composition and how much variation there is in the proportions of the several com- ponents.
Like the higher carbohydrates, the proteins may be broken up into a series of compounds of diminishing complexity by means of several different enzymes acting in succession; these proteo- clastic enzymes have been relatively little studied and but im- perfectly defined. A striking peculiarity of several is that they are active either in strongly acid or strongly alkaline solutions under conditions which render the saccharoclastic enzymes in- operative. This difference is called for probably because of the different wav in which the units are linked.
without which healthy growth is impossible. These indispensable agents or advitants may easily be destroyed in cooking and by preserving foods. Thus infants fed on boiled milk alone rapidly develop symptoms of scurvy, but the addition of a little orange or turnip juice is sufficient to meet the deficiency. To explain such facts and a multitude of similar observations is very difficult, yet the discovery of the explanation is of vital importance, as vast quantities of poor food might have its full value restored, if the deficit in advitant could be made good.
The advitants can scarcely be enzymic agents, as in most cases they withstand more heating than would an enzyme: it is true the antiscorbutic advitant in milk and fresh vegetables is destroyed by heating or drying, but in orange juice survives boiling.
The alkaloid adrenaline, produced constantly in minute pro- portion, is known to be a regulant of the arterial system in the
