The method of obtaining crystalline insulin by the brucine method has been discussed and various modifications which were resorted to with some preparations have been pointed out.
Crystalline insulin obtained by the brucine method has been recrystallized without the use of brucine, retaining its high activity.
Insulin has been crystallized directly from crude insulin without the use of brucine.
Crystalline insulin has also been obtained without the use of ammonia
Cystine and tyrosine have been isolated from hydrolyzed crystalline insulin, proving the presence of these amino acids in the insulin molecule.
A new method for separating cystine from tyrosine has been worked out.
By means of the Sullivan method the amount of cystine has been estimated, the value obtained accounting for most of the sulfur present.
Evidence has been found for the presence of another sulfur compound which gives the Folin-Looney reaction but not the Sullivan reaction. This may be an extremely difficultly hydrolysable dipeptide of cystine or another disulfide compound.
The tyrosine content has been determined by the Folin-Looney colorimetric reaction.