A sensitive diffusion plate assay for screening inhibitors of protease activity in plant cell fractions

Proteolytic activity was detected, using a sensitive radial diffusion plate assay, in the plasma membrane fractions of corn (Zea mays L.) roots and from roots of several other plant species. The proteases could be effectively inhibited in corn with phenylmethane sulfonyl fluoride or chymostatin. Protease activity of oat roots, however, was not significantly reduced by these inhibitors. The results of diffusion plate assay were confirmed with the less sensitive azocasein assay using crude cell homogenates. Chymostatin and phenylmethane sulfonyl fluoride were effective in preventing protease degradation of polypeptides as revealed by electrophoresis. The diffusion plate assay uses a permanent support for a 0.75 millimeter thick agarose slab containing 200 micrograms per milliliter casein. By staining the fixed and dried gel with Coomassie blue R-250, proteolytic activity was visualized as a cleared area around the sample well with a detection limit of about 0.3 nanograms trypsin. The diffusion plate assay should prove useful for screening inhibitors of proteases where limited amounts of material are available, such as with plant cell fractions or highly purified proteins.