Subtilisin inhibitors in Canavalia and Vicia faba seeds. A comparative study

Subtilisin isoinhibitors (SI) were isolated from jack bean (Canavalia ensiformis L) and broad bean (Vicia faba L) seeds. Jack beans contain three isoinhibitors (pI 6.6, 6.3 and 6.0) that constitute 0.021 g per 100 g of dry seeds, while the two active proteins from broad beans (pI 5.7 and 5.1) represent 0.028%. The molecular weight, determined by gel filtration, is around 8000 D in both legumes. Large variations in specific activity of SI against subtilisin Carlsberg were detected in six species of the Canavalia genus, whereas only slight changes were found among six cultivars of C ensiformis. Antibodies raised against SI isolated from jack beans are specific for SI from different varieties of this and other species of the Canavalia genus. However, they do not recognise SI from broad beans and other legume seeds tested. In broad beans the variability between cultivars is significant. Cv Canaria has two active bands and four are detected in the ‚dark’ variety, which is 2.7 times less active than the former. SI are specific towards microbial serine proteases, showing high affinity for proteinase K. No appreciable activity against animal proteases or plant thiol enzymes can be detected. SI are also inactive against 4 alpha‐amylases of different origins. Reversible limited proteolysis of the reactive site bond suggests that SI interact with subtilisin by the 'standard mechanism' usually accepted for the inhibition of trypsin by its proteinaceous inhibitors.