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On the Isolation and Characterization of a C-S-Lyase Preparation from Leek,Allium porrum.

The C-S-lyase protein from leek, ALLIUM PORRUM L., has been purified and characterized. The molecular mass of the native protein was determined with M (r) = 100 000, including two similar subunits, M (r) = 50 000. The tendency of the native protein to form a trimer, M (r) = 300 000, could be supported. The isoelectric point of the enzyme turned out to be close to pH 7.5. The C-S-lyase reaction revealed a wide pH optimum, in the range of 6.1 to 6.9. The temperature optimum was found to be at 41 degrees C. Pure (+)- and (-)-isomers of S-alk(en)yl- L-cysteine sulfoxides were inserted as the substrates. The highest turnover rate was achieved with (+)- S-allyl- L-cysteine sulfoxide (alliin). (+)- S-Propyl- L-cysteine sulfoxide (PCSO) exhibited the lowest K (m) value. Activation energies for the cleavage of the substrates were determined to be 23 kJ/mol for (+)- S-methyl- L-cysteine sulfoxide (MCSO), 38 kJ/mol for (-)-MCSO, 28 kJ/mol for (+)-alliin, and 54 kJ/mol for (+)-PCSO. On the basis of studies with specific inhibitors, pyridoxal 5'-phosphate was found to be part of the A. PORRUM C-S-lyase protein as a cofactor. Competitive inhibitory effects were observed with L-cysteine and related compounds.