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.