Partial Purification and Characterization of Endoproteinases from Senescing Barley Leaves 1

AUTOR(ES)

Miller, Bruce L.

RESUMO

Two major endoproteinases were purified from senescing primary barley leaves. The major enzyme (EP1) appeared to be a thiol proteinase and accounted for about 85% of the total proteolytic activity measured in vitro. This proteinase was purified 5,800-fold and had a molecular weight of 28,300. It was highly unstable in the absence of dithiothreitol or at a pH greater than 7.5. Leupeptin, at a concentration of 10 micromolar, inhibited this enzyme 100%. A second proteinase (EP2) was purified approximately 50-fold and had a molecular weight of 67,000. It was inhibited 20% by 1 millimolar dithiothreitol and 50% by 1 millimolar phenylmethyl sulfonylfluoride. EP2 contributed about 15% of the total proteolytic activity measured in vitro. Both proteinases hydrolyzed a variety of artificial and protein substrates, and both had pH optima of 5.5 to 5.7 when either azocasein or [14C]ribulose-1,5-bisphosphate carboxylase ([14C]RuBPCase) was the substrate. The thiol endoproteinase hydrolyzed azocasein linearly but hydrolyzed [14C]RuBPCase biphasically. A third endoproteinase (EP3), not detected by standard proteolytic assays, was observed when [14C]RuBPCase was the substrate.

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