Catabolism of Cyanogenic Glycosides by Purified Vicianin Hydrolase from Squirrel's Foot Fern ( Davallia Trichomanoides Blume)

Abstract

Vicianin hydrolase, which catalyzes the hydrolysis of vicianin (K(m), 4.9 millimolar) to (R)-mandelonitrile and vicianose at an optimum pH of 5.5, was extensively purified from the young fronds and fiddleheads of the squirrel's foot fern (Davallia trichomanoides Blume) using DEAE-cellulose and Ultrogel HA chromatography. The native molecular weight of the enzyme was 340,000, and the isoelectric point was 4.6 to 4.7. SDS-PAGE analysis yielded three polypeptides with molecular weights of 56,000, 49,000, and 32,500. The enzyme hydrolyzed only a narrow range of glycosides and, among cyanogenic glycosides, exhibited a strict requirement for (R)-epimers and a preference for disaccharides over monosaccharides. (R)-Amygdalin, (R)-prunasin and p-nitrophenyl-beta-d-glucoside were hydrolyzed at 27, 14, and 3%, respectively, of the rate of vicianin hydrolysis. Mixed substrate studies showed that (R)-vicianin, (R)-prunasin, and p-nitrophenyl-beta-d-glucoside competed for the same active site. The enzyme was significantly inhibited by castanospermine, delta-gluconolactone, and p-chloromercuriphenylsulfonate. Failure to recognize concanavalin A-Sepharose 4B and to stain with periodic acid-Schiff reagent indicated that the enzyme was not a glycoprotein.