Purification, characterization and partial amino acid sequencing of two new aspartic proteinases from fresh flowers of Cynara cardunculus L

The glycosylation of the aspartic proteinases from barley (Hordeum vulgare L.) and cardoon (Cynara cardunculus L.)

Plant aspartic proteinases characterised at the molecular level contain one or more consensus N-glycosylation sites [Runeberg-Roos, P., Tŏrmäkangas, K. & Ostman, A. (1991) Eur. J. Biochem. 202, 1021-1027; Asakura, T., Watanabe, H., Abe, K. & Arai, S. (1995) Eur. J. Biochem, 232, 77-83; Veríssimo, P., Faro, C., Moir, A. J. G., Lin, Y., Tang, J. & Pires, E. (1996) Eur. J. Biochem. 235, 762-768]. We found that the glycosylation sites are occupied for the barley (Hordeum vulgare L.) aspartic proteinase (Asn333) and the cardoon (Cynara cardunculus L.) aspartic proteinase, cardosin A (Asn70 and Asn363). The oligosaccharides from each site were released from peptide pools by enzymatic hydrolysis with peptide-N-glycanase A or by hydrazinolysis and their structures were determined by exoglycosidase sequencing combined with matrix-assisted laser desorption/ionization time of flight mass spectrometry. It was observed that 6% of the oligosaccharides from the first glycosylation site of cardosin A are of the oligomannose type. Modified type glycans with proximal Fuc and without Xyl account for about 82%, 14% and 3% of the total oligosaccharides from the first and the second glycosylation sites of cardosin A and from H. vulgare aspartic proteinase, respectively. Oligosaccharides with Xyl but without proximal Fuc were only detected in the latter proteinase (4%). Glycans with proximal Fuc and Xyl account for 6%, 86% and 92% of total oligosaccharides from the first and second glycosylation sites of cardosin A and from H. vulgare aspartic proteinase, respectively.

Action on bovine alpha s1-casein of cardosins A and B, aspartic proteinases from the flowers of the cardoon Cynara cardunculus L

The cleavage of purified bovine alpha s1-casein separately by cardosin A and cardosin B, two distinct milk-clotting aspartic proteinases (APs) present in the stigmas of the plant Cynara cardunculus L., was studied. Casein digestion peptides were separated either by SDS-PAGE or by reverse-phase HPLC, and their N-terminal amino acid sequences were subsequently determined by automated Edman degradation, thus identifying the cleavage sites. Results showed that both enzymes exert a similar but distinct action on bovine alpha s1-casein. In common they have the preference for the bond Phe23-Phe24, and the cleavage of Trp164-Tyr165 and Phe153-Tyr154. Cardosin A also cleaves the bond Tyr165-Tyr166, whereas Cardosin B cleaves an extra type of bond, Phe150-Arg151, 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 bovine alpha s1-casein, both cardosins proved to have a broader specificity towards this particular substrate due to a higher ability to cleave bonds between residues with large hydrophobic side-chains.