Characterization of the Proteinase that Initiates the Degradation of the Trypsin Inhibitor in Germinating Mung Beans (Vigna radiata) 1

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The proteinase (proteinase F) responsible for the initial proteolysis of the mung bean (Vigna radiata) trypsin inhibitor (MBTI) during germination has been purified 1400-fold from dry beans. The enzyme acts as an endopeptidase, cleaving the native inhibitor, MBTI-F, to produce the first modified inhibitor form, MBTI-E. The cleavage of the Asp76-Lys77 peptide bond of MBTI-F occurs at a pH optimum of 4.5, with the tetrapeptide Lys-Asp-Asp-Asp being released. Proteinase F exhibited no activity against the modified inhibitor forms MBTI-E and MBTI-C. Vicilin, the major storage protein of the mung bean, does not serve as a substrate for proteinase F between pH 4 and 7. Proteinase F is inhibited by phenylmethylsulfonyl fluoride, chymostatin, p-hydroxymercuribenzoate, and p-chlorophenylsulfonate, but not by iodoacetate and CuCl2. It is not activated by dithiothreitol, and is stable for extended periods of time (10 months, 4°C, pH 4.0) in the absence of reducing agents. An apparent molecular weight of 65,000 was found for proteinase F by gel filtration. Subcellular fractionation in glycerol suggests that greater than 85% of the proteinase F activity is found in the protein bodies of the ungerminated mung bean. The same studies indicate that at least 56% of the MBTI of the seed is also localized in the protein bodies.

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