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882 METABOLISM.
in the body, while the nitrogen of the feces (after deducting about 1 gram
on a mixed diet) is a measure of the non-absorbed part of the nitrogen of
the food. The nitrogen of the food, as well as of the excreta, is generally-
determined by Kjeldahl’s method.
In the oxidation of the proteins in the organism, their sulphur is oxidized
into sulphuric acid, and on this depends the fact that the elimination of
sulphuric acid by the urine, which in man is but to a small extent derived
from the sulphates of the food, nearly makes equal variations with the
elimination of nitrogen by the urine. If the amount of nitrogen and sul-
phur in the proteins is considered as 16 per cent and 1 per cent respectively,
then the proportion between the nitrogen of the proteins and the sulphuric
acid, H2SO4, produced by their combustion is in the ratio 5.2 : 1, or about
the same as in the urine (see page 765). The determination of the quantity
of sulphuric acid eliminated in the urine gives us an important means of
controlling the extent of the transformation of proteins, and such a con-
trol is especially important in cases in which it is expected to study the
action of certain nitrogenous non-albuminous bodies on the metabolism
of proteins, or to decide the question whether a true protein combustion
and not only a washing out of the nitrogenous products of metabolism
from the tissues is taking place. A determination of the nitrogen alone
is naturally not sufficient in such cases. A perfectly positive measure
of the protein catabolism cannot be made from the sulphuric acid of the
urine, as the various protein substances have a rather variable sulphur
content, and on the other hand also a variable quantity of the sulphur
in the urine exists as so-called neutral sulphur.
In metabolism experiments the total sulphur of the urine as well as the
feces must be determined, and it may also be of importance to determine
the relation between the sulphuric-acid sulphur and the neutral sulphur
of the urine. The elimination of the sulphur originating from the proteins
does not, according to v. Wendt, Hamalainen and Helme and Ch.
Wolff x
always run parallel with the protein nitrogen, and for the
white of egg the maxima of the elimination curves may indeed be
separated during a period of twenty-four hours (Wolff). The sulphur
is more quickly eliminated than the nitrogen, and this behavior of sul-
phur gives in certain cases a more positive picture of the temporal
catabolism of protein than the nitrogen. This is of importance, as the
elimination of the nitrogen corresponding to a certain amount of protein
requires several days for completion. Falta has also observed that the
chief amount of nitrogen in man on taking different proteins is secreted
with varying rapidity, and the same is true, according to Hamalainen
1
Wendt, Skand. Arch. f. Physiol., 17; Hamalainen and Helme, ibid., 19; Falta,
Deutsch. Arch. f. klin. Med., 86; Ch. G. Wolff, Bioch. Zeitschr., 40.
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