General proteinase assay by formation of SDS-protein gels of proteolyzed substrate proteins

A new approach to the assay of proteinases is described. The method relies on water-insoluble protein substrates, such as gluten and fibrin, which form expanded gels in the presence of sodium dodecyl sulfate (SDS) reagent. Powdered substrate is dispersed in buffer and aliquots are pipetted into long, narrow, 400-microliters tubes made of clear polypropylene. After the addition of enzyme and a period of incubation, a SDS reagent is added, the tubes are centrifuged, and the height of the SDS-protein gel is measured. Reduction of gel height gives a direct measure of enzyme activity. Salt concentration, pH, and incubation times must be consistent for both test and control reactions in order to obtain reproducible results. Examples of proteinases measured by this method are trypsin, chymotrypsin, elastase, pronase, papain, pepsin, an insect (Nysius huttoni) salivary proteinase, and wheat proteinase. The assay could detect enzyme in crude extracts or in purified form. In 1-h incubations, 10 ng of pepsin and elastase or 20 ng of purified insect proteinase could be detected. The assay was simple, fast, economical, and sensitive.