Proteolytic inactivation of leucine aminopeptidase in extracts from bean seeds: effect of solutes

Fröhlich, Viktor; Feller, Urs (1987). Proteolytic inactivation of leucine aminopeptidase in extracts from bean seeds: effect of solutes. Life Science Advances - Plant Physiology, 6, pp. 205-209. Council of Scientific Research Integration

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Cotyledons from dwarf beans (Phaseolus vulgaris.L., var. "Saxa") were extracted at two different stages of germination. Extract from ungerminated seeds (low endopeptidase activity) contained high leucine aminopeptidase activity, which was stable for several hours at pH 5.5 and 30 °C. A rapid inactivation was initiated by adding purified papain or extract from cotyledons of seeds germinated for 8 days (high endopeptidase and low aminopeptidase activities). Leucine aminopeptidase was protected in the presence of sucrose, glucose, fructose, glycine, alanine or serine. Added proteins or polypeptides (competitive substrates for the added endopeptidases) also delayed the inactivation. Only a minor stabilization was measured after adding potassium chloride or lysine, while the inactivation was even accelerated in the presence of magnesium _sulfate or magnesium chloride. The inhibiting effect of magnesium ions was not necessarily linked to the proteolytic inactivation. Our results suggest that the protective effects were due to interactions with the substrate protein (leucine aminopeptidase) and not with the inactivating endopeptidase. Rather high concentrations of the protective solutes were required "in vitro", but it should be considered that a series of substances may be effective simultaneously in intact cells.

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