A modification for increasing the sensitivity of the casein-agar plate assay: a simple semiquantitative assay for thermophilic and mesophilic proteases.

A casein-agar plate assay was used for the quantitative determination of both mesophilic and thermophilic proteases. Because many proteases are thermostable, assay at higher temperatures is possible. The sensitivity of the plate assay increased with temperature, the optimum assay temperature depending on the thermostability of the enzyme (e.g. Thermus protease, 75 degrees C; thermolysin, 65 degrees C; trypsin, 65 degrees C; alpha-chymotrypsin, 45 degrees C). A positive correlation was observed between incubation temperature and the density of the para-casein precipitate, increasing the accuracy of diameter measurement. Using this modified method, thermostable proteases could be assayed at levels well below the limits of detection of other methods (e.g. 40 pg of thermolysin and 300 pg of trypsin detectable at 65 degrees C, a 16-fold increase in the sensitivity for trypsin compared with a conventional plate assay (Fossum, K. (1970) Acta Pathol. Microbiol. Scand. Sect. B 78, 350-361)). The sensitivity of the plate assay could be further increased by the inclusion of some detergents and chaotropic agents in the gel.