184 
THE ENERGY OF THE LIVING PROTOPLASM. 
platinum-black, which renders the alcohol molecule so labile that 
it readily takes up oxygen. We must, look upon both kinds of 
phenomena as katalytic oxidations (cf. the foregoing chapter). (I) 
It is the CH 2 — group in the fatty acids and amido acids, 
and the CH OH— group in ketoses, aldoses, and hydroxylated 
acids that are most easily attacked. The carboxyl-group renders 
the CH OH— group in a molecule much more easily attackable ' 
than does the alcoholic group CH 2 OH. Thus, we see that 
glycerol and mannitol are (at least in the animal) by no means 
easily oxidised, while, e.g., tartaric acid is. The influence of the 
carboxyl-group also becomes evident when we compare the 
bibasic phthalic acid with the monobasic benzoic acid ; the 
latter resists while the former is for the greater part burned up. 
That in the animal the amido-acids (leucin, glycocoll, tyrosin) 
undergo combustion with especial facility, yielding thereby urea 
was demonstrated by Nencki and Schnitzen , as early as 1872, and 
later on by Knieriern, Schmiedeberg, Weiske , and Lewinsky, in 
experiments with asparagin. Nencki declared, therefore, the amido- 
compounds to be the forerunners of urea . (2) This view, which does 
not assume a direct oxidation of dissolved proteids, but a pre¬ 
vious splitting into a group of well known amido-acids, is very 
well supported by observations of Hofmeister , which prove with 
what difficulty peptone, as such, is oxidised in the living animal. 
A rabbit of 1.75 kilo, in weight, discharged, after intravenous injec- 
(1) Nencki and Sicher have tried to determine the extent of oxidation which 
sugar and albumin can undergo if exposed at 36° in alkaline solution to air, and have 
found it to be very slight. How little the protoplasm itself participates directly in 
the oxidation process may be illustrated by the fact that 95 per cent of the matter 
oxidised may be sugar and only 5 per cent proteids ; with bees the percentage of 
the latter is still less. 
(2) The amido-acids from proteids are even more quickly oxidised in the cells 
than the carbohydrates, as must be concluded from investigations by Kumagawa , 
who nourrished dogs with an excess of lean meat and observed that under this con¬ 
dition also the glycogen of the meat was deposited as fat in the animal (Mittheilun- 
gen der medic. Facultät, Tokyo). It is, moreover, of great interest to observe how 
the production of urea is influenced by the chemical constitution : negative groups 
connected with the nitrogen will prevent the formation of urea, as Nencki has shown 
with acetamide, recalling the resistance of hippuric acid. But taurin and sarkosin 
are also oxidised with difficulty, reappearing as uramido-compounds in the urine 
(Salkowski ). Urea must be considered as a synthetical product from carbamic acid 
and ammonia (Drechsel and Abel; Nencki and Hahn). 
