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

Abstract

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 °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.

Key words

Allium porrum - Alliaceae - leek - C-S-lyase - alliin - cysteine derivatives