460 TRANSACTIONS OF SECTION B. 
structure that it must be supposed that they each fulfil somewhat different 
functions in building up the tissues of the body. It thus becomes important 
to see that each is supplied in the proper proportions required by the body. 
Further, the analytical results point to the impossibility of entirely sub- 
stituting for a diet composed of one kind of protein—for example, meat 
—another diet composed, let us say, of nuts, since the two proteins, though 
nade up of the same structural units, contain these in entirely different 
proportions. 
Under the influence of the digestive enzymes the proteins can be con- 
verted into the same amino acids as are obtained when mineral acids are 
used to effect hydrolysis. If, however, the process of ‘ breaking-down’ be 
stopped before completion, compounds in which two or more amino acids 
are joined together—the so-called polypeptides—can be isolated. 
It remains now for the chemist and physiologist to ascertain the 
precise function and significance of each amino acid in metabolism; how 
far they may take the place of one another or may be absent without 
injurious effects; further, to what extent each is concerned in the main- 
tenance of a particular tissue. Judging from the great variety of proteins 
consumed in a normal diet, it is obvious that every one of the units at 
present known to us must be of importance; a diet composed of one 
source of protein only or one which is too monotonous in character, even if 
derived from three or four different sources, is certainly to be condemned. 
Hither will lead to the presence of an excess of particular amino acids and 
a deficiency of others perhaps equally important. All the evidence afforded 
by the chemical structure of the proteins goes, in fact, to show that they are 
not equivalent. 
Probably the presence of most, if not all, of the various known amino 
acids and other units of protein is necessary in a food if health is to be 
maintained. In this connection some experiments of Willcock and Hopkins * 
may be cited, in which it was shown that young mice fed with zein as their 
only source of nitrogen soon died. When tryptophane, a unit which is 
absent in zein, was also administered, life was greatly prolonged ; the addi- 
tion of tyrosine—which, however, is a normal constituent of zein-—~had no 
such effect. Apparently the presence of some tryptophane is essential. 
Some recent investigations of Abderhalden,*? which possess considerable 
interest, may be quoted in illustration of the method of attack which will 
no doubt have to be followed in similar cases. As already indicated, it is 
a moot question whether the amino acids can be changed in nature or in 
amount or synthesised from simpler fractions in the organism ; to answer 
it a study has been made of the silkworm, in which the problem is sim- 
plified by the fact that the worm takes no nourishment after it has begun 
to spin. The protein fibroin obtained from raw silk is characterised by 
yielding a large proportion of glycine and alanine and a fair proportion 
of tyrosine when hydrolysed; it is not without interest that both Italian 
and Canton silks are identical as regards the proportions of mono- 
amino acids they contain. Silkworms, when dried and hydrolysed in the 
same manner as fibroin, were found to contain large supplies of the very 
amino acids most abundant in the silk fibroins.*° The worm gives rise to 
cocoon and moth. Continuing the investigation, the moths were likewise 
subjected to analysis. The amount of glycine, alanine and tyrosine from 
them was very sensibly less and the yields of the other amino acids corre- 
spondingly higher than from the cocoons; in fact, the monoamino acids 
? Willcock and Hopkins, J. Physiol. 1906, 35, 83-102. 
Sictaae ie ae Z. Physiol. Chem. 1909, 58, 334-340; ibid. 59, 170-176; 
-238. 
3 It does not, however, necessarily follow that the worm contains ready-formed 
silk protein in the liquid state. 
_— 2 4 eee 
