Proteases of Senescing Oat Leaves: II. Reaction to Substrates and Inhibitors.

Two proteases isolated from senescent oat (Avena sativa) leaves have been subjected to further study. One of these, an acid protease active at pH 4.2, is inhibited by phenylmethylsulfonyl fluoride (PMSF) but not by iodoacetamide (IAc). The other, active at pH 6.6, is inhibited by both PMSF and IAc. These results, together with previously reported evidence that mercaptoethanol stimulates the activity of only the neutral protease, are taken to indicate that the acid protease is probably of the serine type, whereas the neutral enzyme is of the sulfhydryl type. Both enzymes are inhibited by irradiation in the presence of rose bengal, a selective histidine modification reagent. The acid protease was completely unaffected by chelators, but data on the neutral protease were equivocal.All protein substrates tested were attacked by both enzymes, though at strikingly different rates. Characterization of the digestion products, with denatured hemoglobin as substrate, indicated that the acidic enzyme is an endoprotease, while the neutral one is an exoprotease. Evidence is presented that these proteases undergo autolysis in vitro.