Identification and Stability of Aminopeptidases in Extracts From Bean Seeds

Extract from ungerminated bean seeds (Phaseolus vulgaris L. cv. Saxa) was fractionated by gel chromatography on Sephacryl S-200 and by anion exchange chromatography on diethylaminoethyl- Sephacel. Aminopeptidase activities were measured with the following amino acid-p-nitroanilides: phenylalanine, leucine, methionine, proline, alanine, lysine, arginine and glycine. Four forms differing in their substrate specificities were identified: form 1 (liberating alanine, lysine and arginine), form 2 (liberating leucine, methionine, phenylalanine and perhaps also proline), form 3 (liberating glycine) and form 4 (liberating phenylalanine). Form 1 was highly sensitive to 1,10-phenanthroline, while the same concentration of this chelator caused no major effects on the other forms. All aminopeptidases were relatively stable at pH 5.5 in extracts from ungerminated seeds, but the inactivation was accelerated by the addition of cotyledon extract from 8-day-old seedlings. Under such conditions form 3 was most rapidly inactivated followed by forms 1 and 2, while form 4 was less susceptible. The inactivation of the various forms was affected differently by high concentrations of free amino acids. L-Arginine accelerated the inactivation of form 2 and stabilised simultaneously form 3. All forms were protected by L-alanine. Forms 1, 2 and 4 were stabilised by L-serine and by L-proline. Glycine delayed the inactivation of forms 2, 3 and 4.