Proteinase activity in potato plants

Several vegetative tissues of potato plants were screened for proteinase activity. Both endopeptidase and exopeptidase activities were investigated using gelatin and L-amino acid-4-nitroanilides (benzoyl-L-arginine-4-nitroanilide/BAPA, glutaryl-L-phenyl-alanine-4-nitroanilide/GLUPHEPA, alanine-4-nitro-anilide/APA, leucine-4-nitroanilide/LPA, and benzoyl-L-tyrosine-4-nitroanilide/BTPA) as substrates. Leaves and rootes were found to contain the highest levels of endopeptidase activity; lesser activities were detected in flower petals, sprouts, and tubers. Three different types of proteinases, L-BAPAase (serine proteinase), APAase (thiol proteinase), and BTPAase (sensitive to reducing agents), were characterized in various physical and chemical properties. Their temperature optima were determined to be 25° (L-BAPAase) and 40° (BTPAase, APAase) respectively; their pH optimum was between 8.6 and 9.0, their isoelectric points were between pH 4.25 and 6.0, and their molecular weight was estimated 70,000 (L-BAPAase, APAase) and between 150,000-250,000 (BTPAase). The trypsin-like activity against L-BAPA was inhibited by diisopropylfluorophosphate and by tosyllysine-chloromethyl ketone, but not by trypsin inhibitors from potato and legume.