452 LECTURE XIX. 



attacks the molecule. Lecithin is also decomposed into its separate 

 constituents. In short, an intermediate metabolism takes place in the 

 tissues, which is caused by processes perfectly similar to those taking place 

 in the alimentary canal. There the decomposition serves the purpose of 

 converting substances which are naturally foreign to the organism, but 

 are contained in the food, into substances which the tissues can assimilate 

 and incorporate. Hydrolysis is in all cases the preliminary stage to com- 

 bustion. The fact that oxygen itself in any form is not capable of acting 

 directly upon the cell-nutriment, i.e., that it cannot directly ignite this 

 fuel, makes it possible for the cells to satisfy the demands for energy within 

 quite wide limits without regard to external conditions. The cells are 

 able at all times to utilize certain cleavage-products for building up new 

 cell-material while they make use of other less valuable substances merely 

 as fuel. They can adjust their own economy according to their indi- 

 vidual requirements. The breaking down of the nutriment can take 

 place from time to time along different lines, as we explained in the case 

 of sugar. Only at a certain given moment does oxidation ensue. Again, 

 it is remarkable that, as far as we know at present, we meet with specific 

 actions. Not every oxidase is capable of oxidizing tyrosine. Here cer- 

 tain doubts arise as to whether the oxidation takes place exactly as we 

 have represented, or whether, for example, the oxygen given up by the 

 activated oxygenase actually of its own accord attacks the difficultly 

 oxidizable substances without further assistance and consumes it. We 

 meet with objections to such an assumption, especially as there are many 

 classes of compounds known of which only one optical isomer is oxidized, 

 while the other is not. We have already indicated the behavior of the 

 amino acids. If a rabbit is fed with leucine it is chiefly the Z-leucine which 

 is oxidized, while the greater part of the d-leucine which does not occur in 

 albumin is eliminated as such. 1 A great many similar examples are 

 known. We need merely refer to the observations made with carbo- 

 hydrates. C. Neuberg and J. Wohlgemuth 2 injected d-, 1-, and dl- 

 arabinose into rabbits, and found that 7 . 1 per cent of the Z-arabinose, 36 

 per cent of the d-arabinose, and of the dZ-arabinose 31 per cent of dl- 

 arabinose plus 9.6 per cent of d-arabinose were eliminated unchanged. 

 The remander of the material was oxidized. On the other hand, A. Brion 3 

 found that of levo- and mesotartaric acid 93 . 6 to 97 . 3 per cent were oxi- 

 dized, of racemic acid only 58 . 1 to 75 . 3 per cent, and of dextrotartaric 

 acid 70 . 7 to 74 . 4 per cent. We will discuss these facts later. Here they 



1 J. Wohlgemuth: Ber., 38, 2064 (1905). Schittenhelm and Katzenstein: Z. exper. 

 Path. Ther. 2, 560 (1906). Abderhalden and Samuely: Z. physiol. Chem. 47, 346 

 (1906). 



2 'Ber., 34, 1745 (1901), and Z. physiol. Chem. 35, 41 (1902); 37, 530 (1903). 



3 Z. physiol. Chem. 25, 283 (1898). 



