Activities of some peptidases and proteinases in germinating kidney bean, Phaseolus vulgaris

The activities of aminopeptidase (EC 3.4.11), dipeptidase (EC 3.4.13), carboxypeptidase (EC 3.4.16), naphthylamidase (EC 3.4.11) and proteinases (EC 3.4.21) were assayed in extracts from the cotyledons and the axial tissues of resting and germinating kidney beans ( Phaseolus vulgaris L. cv. Processor). The activities of the alkaline peptidases (aminopeptidase hydrolyzing Leu‐Tyr at pH 9.2 and dipeptidase acting on Ala‐Gly at pH 8.5) and naphthylamidases (hydrolyzing Leu‐β‐naphthylamide at pH 6.4) were high in the cotyledons of resting seeds, but decreased during germination. This decrease was faster than the loss of the total nitrogen. On the contrary, the activities of carboxypeptidase (hydrolyzing carbobenzoxy‐Phe‐Ala at pH 5.9) and proteinases (acting on haemoglobin at pH 3.7 and on casein at pH 5.4 and 7.0) were low in the resting seeds, but increased during germination reaching their maximal values when the mobilization of nitrogen was highest. It has been suggested that the breakdown of storage proteins is initiated inside the protein bodies by acid proteinases and carboxypeptidases. Although the activities of the alkaline peptidases and naphthylamidases decreased during germination, these were still relatively high and enough for the completion of the proteolytic breakdown. Thus, it is suggested that, as a final step in a chain of events, the main role for the alkaline peptidases in the cotyledons of germinating seeds is to provide amino acids for the growth of the seedling.