Mandelonitrile lyase from Ximenia americana L.: stereospecificity and lack of flavin prosthetic group.
A mandelonitrile lyase (EC 4.1.2.10) that catalyzes the dissociation of (S)-(-)-mandelonitrile to benzaldehyde and hydrogen cyanide has been purified to apparent homogeneity from leaves of Ximenia americana L. (Olacaceae). The lyase was purified 122-fold with 38% yield by chromatography on carboxymethyl-cellulose and chromatofocusing. The enzyme had a pH optimum of 5.5, with a Km value of 280 microM. Activity toward 4-hydroxy-(R,S)-mandelonitrile was 77% of that observed with the endogenous substrate; no activity was observed toward the aliphatic substrate acetone cyanohydrin. The enzyme was stable at 4 degrees C and at room temperature for at least 1 month. Native and subunit molecular weights of 38,000 and 36,500, respectively, suggest the enzyme is a monomer. The isoelectric point was pH 3.9 as determined by isoelectric focusing. Staining with periodic acid-Schiff and fluorescein-labeled concanavalin A reagents indicate this enzyme is a glycoprotein. In contrast to (R)-mandelonitrile lyases isolated from Prunus species, the Ximenia lyase does not appear to be a flavoprotein. A second enzyme that eluted from the chromatofocusing column at pH 4.0 was also active toward mandelonitrile. However, this form accounted for less than 10% of the total activity, and its specific activity was only 6% of that of the major component. Additional physical and kinetic studies suggested this activity may be due to a nonspecific enzyme that is active toward mandelonitrile.
Department of Biochemistry and Biophysics, University of California, Davis 95616.
G W
GW
Kuroki
E E
EE
Conn
D000446:Aldehyde-Lyases / Q000302:isolation & purification / Q000378:metabolism*; D002850:Chromatography, Gel; D002852:Chromatography, Ion Exchange; D004591:Electrophoresis, Polyacrylamide Gel; D005415:Flavins / Q000032:analysis; D007700:Kinetics; D046911:Macromolecular Substances; D008970:Molecular Weight; D010944:Plants / Q000201:enzymology*
D005415:Flavins; D046911:Macromolecular Substances; D000446:Aldehyde-Lyases; C021541:mandelonitrile lyase