Cyanogenesis in Cassava1

Abstract

In the cyanogenic crop cassava (Manihot esculenta, Crantz), the final step in cyanide production is the conversion of acetone cyanohydrin, the deglycosylation product of linamarin, to cyanide plus acetone. This process occurs spontaneously at pH greater than 5.0 or enzymatically and is catalyzed by hydroxynitrile lyase (HNL). Recently, it has been demonstrated that acetone cyanohydrin is present in poorly processed cassava root food products. Since it has generally been assumed that HNL is present in all cassava tissues, we reinvestigated the enzymatic properties and tissue-specific distribution of HNL in cassava. We report the development of a rapid two-step purification protocol for cassava HNL, which yields an enzyme that is catalytically more efficient than previously reported (Hughes, J., Carvalho, F., and Hughes, M.[1994] Arch Biochem Biophys 311: 496-502). Analyses of the distribution of HNL activity and protein indicate that the accumulation of acetone cyanohydrin in roots is due to the absence of HNL, not to inhibition of the enzyme. Furthermore, the absence of HNL in roots and stems is associated with very low steady-state HNL transcript levels. It is proposed that the lack of HNL in cassava roots accounts for the high acetone cyanohydrin levels in poorly processed cassava food products.

Quantitative Estimation of Hydrogen Cyanide in Fresh and Cooked Tuber Parenchyma (Pulp) of Three Cultivars of Sweet Cassava Cultivars Grown in Some Parts of Benue State, Nigeria

“…However, as earlier reported by Jogensen et al [5], the foremost shortcoming associated with cassava crop are: low protein content, rapid tuber perishability following harvest and high content of cyanogenic glycosides. The cyanogenic glycosides are a group of secondary metabolites in plants that yield cyanide upon enzyamatic metabolism which have been implicated in their role as herbivore deterrents and as transportable forms of reduced nitrogen [6]. Approximately 25 cyanogenic glycosides have been identified and the major ones in edible parts of plants are: amydalin, dhurrin, linamarin, lotaustralin, prunasin and taxiphyllin being present in shoots, leaves and fruits of plants such as: almonds, sorghum, cassava, lima beans, stone fruit and bamboo shoots [1].…”

“…When the vacuole in the cassava cell is ruptured, there is release of linamarin which rapidly hydrolyses through enzymatic catalysis by the enzyme linamarinase to glucose and acetone cyanohydrins while lotaustralin is hydrolysed to a related cyanoydrin and glucose [1] [7]. Since acetone cyanohydrins is an unstable intermediate, it spontaneously decomposes to acetone and hydrogen cyanide; normally at pH > 5.0 or temperatures > 35˚C [1] [6].…”

Chemical Defence and Toxins of Plants

“…Such compartmentation and enzymatic activation of precursors are commonly observed in several different groups of secondary metabolites, 103 which includes the activation of cyanide glycosides by -glucosidase and hydroxynitrile lyase that produce toxic hydrogen cyanide in many plants such as sorghum, clover, and almond 34,35,[37][38][39][40] (Figure 4), the activation of glucosinolates by thioglucosidase that produce toxic isothiocyanates or nitriles in plants belonging to the order Brassicales [43][44][45] (Figure 5), the activation of O-coumaric acid by -glucosidase that produces coumarin in white melilot (Meliotus alba), 103 the activation of phenolics by PPO that produce highly reactive quinones in many plants 55 (Figure 7), and the activation of an iridoid glycoside, oleuropein, by -glucosidase 73 (Figure 12).…”

“…37 In cassava, Manihot esculenta, linamarin and lotaustralin are widely distributed among tissues, 38 whereas linamarase is concentrated in laticifer and latex 39 and hydroxynitrile lyase is concentrated in the cell wall of leaf tissues but absent in the root. 40 As hydrogen cyanide is highly toxic to most animals and insects, cyanide glycoside-borne defense systems are effective against most herbivores. Nonadapted insects and mammals have limited ability to detoxify hydrogen cyanide by means of the enzyme rhodanese or -cyanoalanine synthase, which converts cyanide ion into less toxic thiocyanate.…”

Growth of selected plant species in biosolids-amended mine tailings

“…it can synthesize and release cyanogenic glycosides (White et al, 1998). Hydrolysis of cyanogenic glycosides produces hydrocyanic acid (HCN), which can potentially mobilise Hg and Au in the substrate and induce metal uptake into the plant, thereby making cassava a potential candidate for future phytoextraction studies.…”