METABOLISM. 39 
age and hydration of the proteids in digestion is to render them 
soluble. Undoubtedly this is an important function of the digestive 
fluids, but the fundamental object lies deeper and is found in the 
constitution of the proteids themselves. 
Nature of the Proteids——While we are still very far removed 
from any adequate knowledge of the molecular structure of the 
proteids, a study of the action upon them of various hydrolytic 
agents, and particularly of the proteolytic enzyms of the digestive 
fluids, has shown that they undergo cleavage along certain definite 
lines, giving rise to two series of products known as the hemi- and 
the anti-series. The primary products are the proteoses, or albu- 
moses (hemi and anti). By further action of the ferment these 
give rise to the secondary or deutero-proteoses, and these in turn 
to peptones, while the peptones of the hemi-series, by the further 
action of trypsin, are broken up, as noted, into simpler bodies such 
as aspartic acid, glutaminic acid, and notably tyrosin and leucin. 
The two latter bodies belong to the aromatic series and contain the 
phenyl radicle, which is thus shown to be present in the bodies of 
the hemi group, while it is absent from the anti group. Without 
pursuing the subject further, enough has been said to show that 
the general result of the digestive proteolysis is to break up the pro- 
teid molecule into a considerable number of unlike fragments.* 
Differences in Proteids.—Turning now to another phase of the 
subject, it is a familiar fact that the numerous proteids which have 
been studied differ quite markedly from each other in properties 
and in composition. To instance but a single characteristic differ- 
ence, the investigations of Osborne and his associates at the Connec- 
ticut Agricultural Experiment Station have shown in detail what 
was to a certain extent known before, viz., that the nitrogen con- 
tent of the vegetable proteids is notably higher than that of the 
animal proteids. We can only interpret these differences in com- 
position and properties as the results of differences in molecular 
structure. We may fairly suppose that these differences in struc- 
ture are brought about, in part at least, by differences in the relative 
proportions in the proteid molecule of the several molecular group- 
ings whose presence is indicated to us by the results of proteolysis. 
*For a full treatment of the subject, compare Chittenden, Digestive Pro- 
teolysis, 1894. 
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