Proteases and Peptidases of Castor Bean Endosperm: Enzyme Characterization and Changes during Germination

The endosperm of castor bean seeds (Ricinus communis L.) contains two -SH-dependent aminopeptidases, one hydrolyzing l-leucine-beta-naphthylamide optimally at pH 7.0, and the other hydrolyzing l-proline-beta-naphthylamide optimally at pH 7.5. After germination the endosperm contains in addition an -SH-dependent hemoglobin protease, a serine-dependent carboxypeptidase, and at least two -SH-dependent enzymes hydrolyzing the model substrate alpha-N-benzoyl-dl-arginine-beta-naphthylamide (BANA). The carboxypeptidase is active on a variety of N-carbobenzoxy dipeptides, especially N-carbobenzoxy-L-phenylalanine-l-alanine and N-carbobenzoxy-l-tyrosine-l-leucine. The pH optima for the protease, carboxypeptidase, and BANAase acivities are 3.5 to 4.0, 5.0 to 5.5, and 6 to 8, respectively.The two aminopeptidases increased about 4-fold in activity during the first 4 days of growth, concurrent with the period of rapid depletion of storage protein. Activities then declined as the endosperm senesced, but were still evident after 6 days. Senescence was complete by day 7 to 8. Hemoglobin protease, carboxypeptidase, and BANAase activities appeared in the endosperm at day 2 to 3, and reached peak activity at day 5 to 6.The data indicate that the aminopeptidases are involved in the early mobilization of endosperm storage protein, whereas protease, carboxypeptidase, and BANAase may take part in later turnover and/or senescence.