Enzymic proteolysis

Summary

A study has been made of the rate of liberation of cystine and amino nitrogen from eight proteins during successive digestion by pepsin and trypsin.

Assuming the specificity of the Sullivan reaction it can be concluded that no free cystine is present in peptic digests.

The same reaction shows that cystine is rapidly set free during the action of trypsin. From the vegetable proteins (edestin, gliadin, anacardein and watermelon globulin) cystine is liberated much more rapidly than other amino-acids, practically the whole of the cystine in the protein being present in the free condition at a very early stage of proteolysis. Such complete liberation does not take place with the animal proteins (serum globulin, egg albumin and fibrin) ; with these, rate of cystine splitting approximates more closely to peptide hydrolysis. In the course of tryptic digestion at alkaline pH destruction of cystine takes place, the extent of the destruction varying from protein to protein.

The Folin and Marenzi reagent gives high colour values with both peptic and tryptic digests. These values have no relation to the free cystine present, being frequently in great excess of the total cystine content of the protein. This colour production is due to cystine complexes and not to non-nitrogenous substances as has been previously suggested.