of Edinburgh, Session 1869-70. 
205 
L 
Remarks. 
Cramer believed that cystine was intimately related to the body 
called Serin, C 3 H 7 N0 3 , which is obtained as one of the products 
of the action of alkalies on silk. Serin, when treated with nitrous 
acid, yields glyceric acid, as alanine under the same circumstances 
yields lactic acid, and therefore serin may be looked upon as 
amido-glyceric acid. 
Cramer further believed that cystine was a sulpho-amido-glyceric 
acid, i.e., serin in which hydroxyl has been replaced by HS. 
This supposed relation is exhibited below — 
ch 2 oh 
ch 2 nh 2 
ch 2 nh, 
CHOH 
CHOH 
CHSH 
co 2 h 
co 2 h 
co 2 h 
Glyceric Acid. 
Amido-glyceric Acid 
or Serin. 
Cystine. 
Considering that this relation of cystine to serin really exists, 
some have argued that on treatment with nitrous acid, cystine 
should yield glyceric acid. We do not, however, admit that this 
would really be the case. If we examine the case of sulpho-lactic 
acid, an analogous body to the supposed sulphur derivative of 
serin, we find that, on oxidation, it gives sulpho-propionic acid, 
and therefore we should, in the case of cystine, expect that a 
sulpho-acid would be formed on treatment with nitrous acid, were 
it built up as Cramer supposed. We have uniformly observed, 
during the course of our experiments, that, however carefully we 
attempted to regulate the action of nitrous acid on cystine, or of a 
nitrite on a salt of cystine, the sulphur separated as sulphuric acid 
thus pointing to a material difference in its reactions from what 
we should have expected from its supposed constitution. Although 
we cannot consider our experiments as definitive, we can assert 
that glyceric acid is not a product of the action of nitrous acid, 
and we venture to predict that, in all probability, cystine will be 
found to be related to pyruvic acid — to be an amido-sulpho-pyruvic 
acid. We base this supposition on the near approach of the 
analyses of the silver salt of the acid obtained by the action of 
nitrous acid on cystine, to the composition of a pyruvate, and on 
the general character of the oily acid produced. 
We intend to pursue this subject further. 
2 D 
VOL. VII. 
