REGULATION OF CYANOGENESIS IN CASSAVA
Linamarin, the predominant cyanogenic glucoside in cassava (Manihot esculenta Crantz), is present in all organs of the plant. The hydrolysis of linamarin by the -glucosidase, linamarase, results in the production of acetone cyanohydrin. Acetone cyanohydrin can decompose to hydrogen cyanide and acetone spontaneously or enzymatically by hydroxynitrile lyase. It had been assumed that linamarin synthesis was restricted to the leaves, and that linamarin present in roots was transported from the leaves. However, we demonstrate that root tissues are capable of synthesizing linamarin at rates comparable to those in leaves. In addition, the presence of linamarase in the apoplast precludes an apoplastic transport of linamarin, unless linamarin is modified (glucosylated) to form a non-hydrolyzable compound prior to transport. Labeling experiments using a radioactive precursor (valine) to linamarin indicate that linamarin is not glucosylated. Furthermore, it has been demonstrated that linamarin is stored in the vacuole. In addition, we demonstrate that hydroxynitrile lyase, like linamarase, is localized in the apoplast. Hydroxynitrile lyase was purified to apparent homogeneity. Hydroxynitrile lyase has optimal activity at a pH of 5.0 and a temperature of 30°C. Hydroxynitrile lyase is quite stable similar to linamarase. Analyses of the stability of acetone cyanohydrin and activity of hydroxynitrile lyase suggest that cassava processing at pH greater than 5.0 or in the presence of additional hydroxynitrile lyase would reduce the level of this cyanogen in food products.
White, W. L.B., McMahon, J. M. and Sayre, R. T. (1994). REGULATION OF CYANOGENESIS IN CASSAVA. Acta Hortic. 375, 69-78
Cyanide, linamarin, linamarase, acetone cyanohydrin, hydroxynitrile lyase