Proteinases and enzyme stability in crude extracts of castor bean endosperm

The stability of catalase, fumarase, and isocitrate lyase from deliberately broken organelles in crude extracts from endosperm tissue of castor bean seedlings has been examined. These enzymes are relatively stable at 2 C in extracts from endosperm of 2-day seedlings, but rapid losses of activity occur in extracts from older seedlings. These losses are shown to be brought about by the thiol-proteinase present in the extracts. The inclusion of 35% glycerol prevented the loss of catalase, fumarase, and isocitrate lysase activity, and various inhibitors of proteinases afforded limited protection. The most striking protectant was leupeptin, an inhibitor of serine and thiol-proteinases. Leupeptin completely inhibited the loss of activity of the three enzymes in crude extracts and improved yields when included in the grinding medium.