ACTION ON BOVINE ALPHA(S1)-CASEIN OF CARDOSIN-A AND CARDOSIN-B, ASPARTIC PROTEINASES FROM THE FLOWERS OF THE CARDOON CYNARA-CARDUNCULUS L

The cleavage of purified bovine alpha(s1)-casein separately by cardosi n A and cardosin B, two distinct milk-clotting aspartic proteinases (A Ps) present in the stigmas of the plant Cynara cardunculus L., was stu died. Casein digestion peptides were separated either by SDS-PAGE or b y reverse-phase HPLC, and their N-terminal amino-acid sequences were s ubsequently determined by automated Edman degradation, thus identifyin g the cleavage sites. Results showed that both enzymes exert a similar but distinct action on bovine alpha(s1)-casein. In common they have t he preference for the bond Phe(23)-Phe(24), and the cleavage of Trp(16 4)-Tyr(165) and Phe(153)-Tyr(154). Cardosin A also cleaves the bond Ty r(165)-Tyr(166), whereas Cardosin B cleaves an extra type of bond, Phe (150)-Arg(151), revealing a slightly broader specificity. A model for the action of both enzymes on bovine alpha(s1)-casein is proposed and discussed. In comparison with the reported action of chymosin on bovin e alpha(s1)-casein, both cardosins proved to have a broader specificit y towards this particular substrate due to a higher ability to cleave bonds between residues with large hydrophobic side-chains.