Heinrich Hlasiwetz (1825-75) & Josef Habermann (1841-1914)

On the protein substances: second treatise

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Casein

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The facts presented are the result of eight experiments essentially run exactly in the same way, according to the method described [above]:

Accordingly we consider as proved:

1. That casein yields as breakdown products exclusively:
   1. Glutamic acid,
   2. Aspartic acid,
   3. Leucine,
   4. Tyrosine,
   5. Ammonia.

2. It yields neither carbohydrate nor characteristic derivatives of the same. Contrary to earlier conjectures carbohydrate cannot be concerned in its constitution.

3. It is highly probable that the ammonia which always appears is derived from primary compounds contained in the casein, which at the same time yield aspartic acid and glutamic acid.

   This also would provide a natural explanation for the origin of the so-called 'loosely bound nitrogen' of the protein substances, on which repeated remarks have already been made[1] and the exact quantitative estimation of which O. Nasse[2] only recently undertook again.

   It is the nitrogen of the NH₂ group which comes from compounds like asparagine and glutamine in the form of ammonia, when aspartic acid and glutamic acid are formed.

   Compounds of this kind, which on boiling with acid or alkali lose ammonia and yield these acids with uptake of water, must in general be considered pre-existent in casein and the protein substances.

   Whether these are identical with the customary asparagine and the still to be prepared homologous glutamine, and whether the acids obtained are not products of a molecular re-arrangement and shifting, cannot be decided at present.

4. Glutamic acid is not characteristic exclusively of the plant protein substances as one could be tempted to assume according to the experiments of Kreusler (Journal für pract. Chemie 107, 240), who could not obtain it out of animal material, but it is a constant and in amount a significant breakdown product of all the chief accepted forms of animal protein substances. The maximum yield from casein we obtained was about 29 per cent.

   We have also investigated the breakdown of protein substances according to our method with albumin, legumin and the plant albumin, and the qualitative result was the same as with the casein.

   We further endeavoured to find a quantitative expression for this manner of breakdown; but we have still not succeeded in separating and estimating the amounts of the single breakdown products with the desired exactitude.

   Only this we can state on the grounds of numerous already collected data that the different protein modifications yield different amounts of these products, and it appears to us now already as more than a mere guess that the differences in the properties of the protein modifications are to be sought in a different proportion of the primary atom groups of which they are constituted.

[2]Pflüger's Archiv für Physiologie 6, 589; 7, 139. [original note]